MirBSD manpage: raidctl(8)

RAIDCTL(8)               BSD System Manager's Manual                RAIDCTL(8)


     raidctl - configuration utility for the RAIDframe disk driver


     raidctl [-v] [-afFgrR component] [-BGipPsSu] [-cC config_file]
             [-A [yes | no | root]] [-I serial_number] dev


     raidctl is the user-land control program for raid(4), the RAIDframe disk
     device. raidctl is primarily used to dynamically configure and unconfig-
     ure RAIDframe disk devices. For more information about the RAIDframe disk
     device, see raid(4).

     This document assumes the reader has at least rudimentary knowledge of
     RAID and RAID concepts.

     The device used by raidctl is specified by dev. dev may be either the
     full name of the device, e.g. /dev/rraid0c, or just simply raid0 (for

     For several commands (-BGipPsSu), raidctl can accept the word all as the
     dev argument. If all is used, raidctl will execute the requested action
     for all the configured raid(4) devices.

     The command-line options for raidctl are as follows:

     -a component dev
             Add component as a hot spare for the device dev.

     -A yes dev
             Make the RAID set auto-configurable. The RAID set will be au-
             tomatically configured at boot before the root filesystem is
             mounted. Note that all components of the set must be of type RAID
             in the disklabel.

     -A no dev
             Turn off auto-configuration for the RAID set.

     -A root dev
             Make the RAID set auto-configurable, and also mark the set as be-
             ing eligible to contain the root partition. A RAID set configured
             this way will override the use of the boot disk as the root dev-
             ice. In MirBSD however, this only takes place if the kernel is
             configured with "config bsd generic" or "root on raid0". All com-
             ponents of the set must be of type RAID in the disklabel. Note
             that the kernel being booted must currently reside on a non-RAID
             set and, in order to have the root filesystem correctly mounted
             from it, the RAID set must have its 'a' partition (aka
             raid[0..n]a) set up.

     -B dev  Initiate a copyback of reconstructed data from a spare disk to
             its original disk. This is performed after a component has
             failed, and the failed drive has been reconstructed onto a spare

     -c config_file dev
             Configure the RAIDframe device dev according to the configuration
             given in config_file. A description of the contents of
             config_file is given later.

     -C config_file dev
             As for -c, but forces the configuration to take place. This is
             required the first time a RAID set is configured.

     -f component dev
             This marks the specified component as having failed, but does not
             initiate a reconstruction of that component.

     -F component dev
             Fails the specified component of the device, and immediately be-
             gin a reconstruction of the failed disk onto an available hot
             spare. This is one of the mechanisms used to start the recon-
             struction process if a component does have a hardware failure.

     -g component dev
             Get the component label for the specified component.

     -G dev  Generate the configuration of the RAIDframe device in a format
             suitable for use with raidctl -c or -C.

     -i dev  Initialize the RAID device. In particular, (re-write) the parity
             on the selected device. This MUST be done for all RAID sets be-
             fore the RAID device is labeled and before filesystems are creat-
             ed on the RAID device.

     -I serial_number dev
             Initialize the component labels on each component of the device.
             serial_number is used as one of the keys in determining whether a
             particular set of components belong to the same RAID set. While
             not strictly enforced, different serial numbers should be used
             for different RAID sets. This step MUST be performed when a new
             RAID set is created.

     -p dev  Check the status of the parity on the RAID set. Displays a status
             message, and returns successfully if the parity is up-to-date.

     -P dev  Check the status of the parity on the RAID set, and initialize
             (re-write) the parity if the parity is not known to be up-to-
             date. This is normally used after a system crash (and before a
             fsck(8)) to ensure the integrity of the parity.

     -r component dev
             Remove the spare disk specified by component from the set of
             available spare components.

     -R component dev
             Fails the specified component, if necessary, and immediately be-
             gins a reconstruction back to component. This is useful for
             reconstructing back onto a component after it has been replaced
             following a failure.

     -s dev  Display the status of the RAIDframe device for each of the com-
             ponents and spares.

     -S dev  Check the status of parity re-writing, component reconstruction,
             and component copyback. The output indicates the amount of pro-
             gress achieved in each of these areas.

     -u dev  Unconfigure the RAIDframe device.

     -v      Be more verbose. For operations such as reconstructions, parity
             re-writing, and copybacks, provide a progress indicator.

Configuration file

     The format of the configuration file is complex, and only an abbreviated
     treatment is given here. In the configuration files, a '#' indicates the
     beginning of a comment.

     There are 4 required sections of a configuration file, and 2 optional
     sections. Each section begins with a 'START', followed by the section
     name, and the configuration parameters associated with that section. The
     first section is the 'array' section, and it specifies the number of
     rows, columns, and spare disks in the RAID set. For example:

           START array
           1 3 0

     indicates an array with 1 row, 3 columns, and 0 spare disks. Note that
     although multi-dimensional arrays may be specified, they are NOT support-
     ed in the driver.

     The second section, the 'disks' section, specifies the actual components
     of the device. For example:

           START disks

     specifies the three component disks to be used in the RAID device. If any
     of the specified drives cannot be found when the RAID device is config-
     ured, then they will be marked as 'failed', and the system will operate
     in degraded mode. Note that it is imperative that the order of the com-
     ponents in the configuration file does not change between configurations
     of a RAID device. Changing the order of the components will result in
     data loss if the set is configured with the -C option. In normal cir-
     cumstances, the RAID set will not configure if only -c is specified, and
     the components are out-of-order.

     The next section, which is the 'spare' section, is optional, and, if
     present, specifies the devices to be used as 'hot spares' -- devices
     which are on-line, but are not actively used by the RAID driver unless
     one of the main components fail. A simple 'spare' section might be:

           START spare

     for a configuration with a single spare component. If no spare drives are
     to be used in the configuration, then the 'spare' section may be omitted.

     The next section is the 'layout' section. This section describes the gen-
     eral layout parameters for the RAID device, and provides such information
     as sectors per stripe unit, stripe units per parity unit, stripe units
     per reconstruction unit, and the parity configuration to use. This sec-
     tion might look like:

           START layout
           # sectPerSU SUsPerParityUnit SUsPerReconUnit RAID_level
           32 1 1 5

     The sectors per stripe unit specifies, in blocks, the interleave factor;
     i.e. the number of contiguous sectors to be written to each component for
     a single stripe. Appropriate selection of this value (32 in this example)
     is the subject of much research in RAID architectures. The stripe units
     per parity unit and stripe units per reconstruction unit are normally
     each set to 1. While certain values above 1 are permitted, a discussion
     of valid values and the consequences of using anything other than 1 are
     outside the scope of this document. The last value in this section (5 in
     this example) indicates the parity configuration desired. Valid entries

     0     RAID level 0. No parity, only simple striping.

     1     RAID level 1. Mirroring. The parity is the mirror.

     4     RAID level 4. Striping across components, with parity stored on the
           last component.

     5     RAID level 5. Striping across components, parity distributed across
           all components.

     There are other valid entries here, including those for Even-Odd parity,
     RAID level 5 with rotated sparing, Chained declustering, and Interleaved
     declustering, but as of this writing the code for those parity operations
     has not been tested with OpenBSD.

     The next required section is the 'queue' section. This is most often
     specified as:

           START queue
           fifo 100

     where the queuing method is specified as FIFO (First-In, First-Out), and
     the size of the per-component queue is limited to 100 requests. Other
     queuing methods may also be specified, but a discussion of them is beyond
     the scope of this document.

     The final section, the 'debug' section, is optional. For more details on
     this the reader is referred to the RAIDframe documentation discussed in
     the HISTORY section. See EXAMPLES for a more complete configuration file


     It is highly recommended that before using the RAID driver for real
     filesystems that the system administrator(s) become quite familiar with
     the use of raidctl, and that they understand how the component recon-
     struction process works. The examples in this section will focus on con-
     figuring a number of different RAID sets of varying degrees of redundan-
     cy. By working through these examples, administrators should be able to
     develop a good feel for how to configure a RAID set, and how to initiate
     reconstruction of failed components.

     In the following examples 'raid0' will be used to denote the RAID device.
     '/dev/rraid0c' may be used in place of 'raid0'.

Initialization and Configuration

     The initial step in configuring a RAID set is to identify the components
     that will be used in the RAID set. All components should be the same
     size. Each component should have a disklabel type of FS_RAID, and a typi-
     cal disklabel entry for a RAID component might look like:

           f:  1800000  200495     RAID              # (Cyl.  405*- 4041*)

     While FS_BSDFFS (e.g. 4.2BSD) will also work as the component type, the
     type FS_RAID (e.g. RAID) is preferred for RAIDframe use, as it is re-
     quired for features such as auto-configuration. As part of the initial
     configuration of each RAID set, each component will be given a 'component
     label'. A 'component label' contains important information about the com-
     ponent, including a user-specified serial number, the row and column of
     that component in the RAID set, the redundancy level of the RAID set, a
     'modification counter', and whether the parity information (if any) on
     that component is known to be correct. Component labels are an integral
     part of the RAID set, since they are used to ensure that components are
     configured in the correct order, and used to keep track of other vital
     information about the RAID set. Component labels are also required for
     the auto-detection and auto-configuration of RAID sets at boot time. For
     a component label to be considered valid, that particular component label
     must be in agreement with the other component labels in the set. For ex-
     ample, the serial number, 'modification counter', number of rows and
     number of columns must all be in agreement. If any of these are dif-
     ferent, then the component is not considered to be part of the set. See
     raid(4) for more information about component labels.

     Once the components have been identified, and the disks have appropriate
     labels, raidctl is then used to configure the raid(4) device. To config-
     ure the device, a configuration file which looks something like:

           START array
           # numRow numCol numSpare
           1 3 1

           START disks

           START spare

           START layout
           # sectPerSU SUsPerParityUnit SUsPerReconUnit RAID_level_5
           32 1 1 5

           START queue
           fifo 100

     is created in a file. The above configuration file specifies a RAID 5 set
     consisting of the components /dev/sd1e, /dev/sd2e, and /dev/sd3e, with
     /dev/sd4e available as a 'hot spare' in case one of the three main drives
     should fail. A RAID 0 set would be specified in a similar way:

           START array
           # numRow numCol numSpare
           1 4 0

           START disks

           START layout
           # sectPerSU SUsPerParityUnit SUsPerReconUnit RAID_level_0
           64 1 1 0

           START queue
           fifo 100

     In this case, devices /dev/sd10e, /dev/sd11e, /dev/sd12e, and /dev/sd13e
     are the components that make up this RAID set. Note that there are no hot
     spares for a RAID 0 set, since there is no way to recover data if any of
     the components fail.

     For a RAID 1 (mirror) set, the following configuration might be used:

           START array
           # numRow numCol numSpare
           1 2 0

           START disks

           START layout
           # sectPerSU SUsPerParityUnit SUsPerReconUnit RAID_level_1
           128 1 1 1

           START queue
           fifo 100

     In this case, /dev/sd20e and /dev/sd21e are the two components of the
     mirror set. While no hot spares have been specified in this configura-
     tion, they easily could be, just as they were specified in the RAID 5
     case above. Note as well that RAID 1 sets are currently limited to only 2
     components. At present, n-way mirroring is not possible.

     The first time a RAID set is configured, the -C option must be used:

           # raidctl -C raid0.conf raid0

     where 'raid0.conf' is the name of the RAID configuration file. The -C
     forces the configuration to succeed, even if any of the component labels
     are incorrect. The -C option should not be used lightly in situations
     other than initial configurations, as if the system is refusing to con-
     figure a RAID set, there is probably a very good reason for it. After the
     initial configuration is done (and appropriate component labels are added
     with the -I option) then raid0 can be configured normally with:

           # raidctl -c raid0.conf raid0

     When the RAID set is configured for the first time, it is necessary to
     initialize the component labels, and to initialize the parity on the RAID
     set. Initializing the component labels is done with:

           # raidctl -I 112341 raid0

     where '112341' is a user-specified serial number for the RAID set. This
     initialization step is required for all RAID sets. Also, using different
     serial numbers between RAID sets is strongly encouraged, as using the
     same serial number for all RAID sets will only serve to decrease the use-
     fulness of the component label checking.

     Initializing the RAID set is done via the -i option. This initialization
     MUST be done for all RAID sets, since among other things it verifies that
     the parity (if any) on the RAID set is correct. Since this initialization
     may be quite time-consuming, the -v option may be also used in conjunc-
     tion with -i:

           # raidctl -iv raid0

     This will give more verbose output on the status of the initialization:

           Initiating re-write of parity
           Parity Re-write status:
            10% |****                                   | ETA:    06:03 /

     The output provides a 'Percent Complete' in both a numeric and graphical
     format, as well as an estimated time to completion of the operation.

     Since it is the parity that provides the 'redundancy' part of RAID, it is
     critical that the parity is correct as much as possible. If the parity is
     not correct, then there is no guarantee that data will not be lost if a
     component fails.

     Once the parity is known to be correct, it is then safe to perform
     disklabel(8), newfs(8), or fsck(8) on the device or its filesystems, and
     then to mount the filesystems for use.

     Under certain circumstances (e.g. the additional component has not ar-
     rived, or data is being migrated off of a disk destined to become a com-
     ponent) it may be desirable to configure a RAID 1 set with only a single
     component. This can be achieved by configuring the set with a physically
     existing component (as either the first or second component) and with a
     'fake' component. In the following:

           START array
           # numRow numCol numSpare
           1 2 0

           START disks

           START layout
           # sectPerSU SUsPerParityUnit SUsPerReconUnit RAID_level_1
           128 1 1 1

           START queue
           fifo 100

     /dev/sd0e is the real component, and will be the second disk of a RAID 1
     set. The component /dev/sd6e, which must exist, but have no physical dev-
     ice associated with it, is simply used as a placeholder. Configuration
     (using -C and -I 12345 as above) proceeds normally, but initialization of
     the RAID set will have to wait until all physical components are present.
     After configuration, this set can be used normally, but will be operating
     in degraded mode. Once a second physical component is obtained, it can be
     hot-added, the existing data mirrored, and normal operation resumed.

Maintenance of the RAID set

     After the parity has been initialized for the first time, the command:

           # raidctl -p raid0

     can be used to check the current status of the parity. To check the pari-
     ty and rebuild it necessary (for example, after an unclean shutdown) the

           # raidctl -P raid0

     is used. Note that re-writing the parity can be done while other opera-
     tions on the RAID set are taking place (e.g. while doing an fsck(8) on a
     filesystem on the RAID set). However: for maximum effectiveness of the
     RAID set, the parity should be known to be correct before any data on the
     set is modified.

     To see how the RAID set is doing, the following command can be used to
     show the RAID set's status:

           # raidctl -s raid0

     The output will look something like:

                      /dev/sd1e: optimal
                      /dev/sd2e: optimal
                      /dev/sd3e: optimal
                      /dev/sd4e: spare
           Parity status: clean
           Reconstruction is 100% complete.
           Parity Re-write is 100% complete.
           Copyback is 100% complete.

     This indicates that all is well with the RAID set. Of importance here are
     the component lines which read 'optimal', and the 'Parity status' line
     which indicates that the parity is up-to-date. Note that if there are
     filesystems open on the RAID set, the individual components will not be
     'clean' but the set as a whole can still be clean.

     The -v option may be also used in conjunction with -s:

           # raidctl -sv raid0

     In this case, the components' label information (see the -g option) will
     be given as well:

                      /dev/sd1e: optimal
                      /dev/sd2e: optimal
                      /dev/sd3e: optimal
                      /dev/sd4e: spare
           Component label for /dev/sd1e:
              Row: 0 Column: 0 Num Rows: 1 Num Columns: 3
              Version: 2 Serial Number: 13432 Mod Counter: 65
              Clean: No Status: 0
              sectPerSU: 32 SUsPerPU: 1 SUsPerRU: 1
              RAID Level: 5  blocksize: 512 numBlocks: 1799936
              Autoconfig: No
              Last configured as: raid0
           Component label for /dev/sd2e:
              Row: 0 Column: 1 Num Rows: 1 Num Columns: 3
              Version: 2 Serial Number: 13432 Mod Counter: 65
              Clean: No Status: 0
              sectPerSU: 32 SUsPerPU: 1 SUsPerRU: 1
              RAID Level: 5  blocksize: 512 numBlocks: 1799936
              Autoconfig: No
              Last configured as: raid0
           Component label for /dev/sd3e:
              Row: 0 Column: 2 Num Rows: 1 Num Columns: 3
              Version: 2 Serial Number: 13432 Mod Counter: 65
              Clean: No Status: 0
              sectPerSU: 32 SUsPerPU: 1 SUsPerRU: 1
              RAID Level: 5  blocksize: 512 numBlocks: 1799936
              Autoconfig: No
              Last configured as: raid0
           Parity status: clean
           Reconstruction is 100% complete.
           Parity Re-write is 100% complete.
           Copyback is 100% complete.

     To check the component label of /dev/sd1e, the following is used:

           # raidctl -g /dev/sd1e raid0

     The output of this command will look something like:

           Component label for /dev/sd1e:
              Row: 0 Column: 0 Num Rows: 1 Num Columns: 3
              Version: 2 Serial Number: 13432 Mod Counter: 65
              Clean: No Status: 0
              sectPerSU: 32 SUsPerPU: 1 SUsPerRU: 1
              RAID Level: 5  blocksize: 512 numBlocks: 1799936
              Autoconfig: No
              Last configured as: raid0

Dealing with Component Failures

     If for some reason (perhaps to test reconstruction) it is necessary to
     pretend a drive has failed, the following will perform that function:

           # raidctl -f /dev/sd2e raid0

     The system will then be performing all operations in degraded mode, where
     missing data is re-computed from existing data and the parity. In this
     case, obtaining the status of raid0 will return (in part):

                      /dev/sd1e: optimal
                      /dev/sd2e: failed
                      /dev/sd3e: optimal
                      /dev/sd4e: spare

     Note that with the use of -f a reconstruction has not been started. To
     both fail the disk and start a reconstruction, the -F option must be

           # raidctl -F /dev/sd2e raid0

     The -f option may be used first, and then the -F option used later, on
     the same disk, if desired. Immediately after the reconstruction is start-
     ed, the status will report:

                      /dev/sd1e: optimal
                      /dev/sd2e: reconstructing
                      /dev/sd3e: optimal
                      /dev/sd4e: used_spare
           Parity status: clean
           Reconstruction is 10% complete.
           Parity Re-write is 100% complete.
           Copyback is 100% complete.

     This indicates that a reconstruction is in progress. To find out how the
     reconstruction is progressing the -S option may be used. This will indi-
     cate the progress in terms of the percentage of the reconstruction that
     is completed. When the reconstruction is finished the -s option will

                      /dev/sd1e: optimal
                      /dev/sd2e: spared
                      /dev/sd3e: optimal
                      /dev/sd4e: used_spare
           Parity status: clean
           Reconstruction is 100% complete.
           Parity Re-write is 100% complete.
           Copyback is 100% complete.

     At this point there are at least two options. First, if /dev/sd2e is
     known to be good (i.e. the failure was either caused by -f or -F, or the
     failed disk was replaced), then a copyback of the data can be initiated
     with the -B option. In this example, this would copy the entire contents
     of /dev/sd4e to /dev/sd2e. Once the copyback procedure is complete, the
     status of the device would be (in part):

                      /dev/sd1e: optimal
                      /dev/sd2e: optimal
                      /dev/sd3e: optimal
                      /dev/sd4e: spare

     and the system is back to normal operation.

     The second option after the reconstruction is to simply use /dev/sd4e in
     place of /dev/sd2e in the configuration file. For example, the configura-
     tion file (in part) might now look like:

           START array
           1 3 0

           START drives

     This can be done as /dev/sd4e is completely interchangeable with
     /dev/sd2e at this point. Note that extreme care must be taken when chang-
     ing the order of the drives in a configuration. This is one of the few
     instances where the devices and/or their orderings can be changed without
     loss of data! In general, the ordering of components in a configuration
     file should never be changed.

     If a component fails and there are no hot spares available on-line, the
     status of the RAID set might (in part) look like:

                      /dev/sd1e: optimal
                      /dev/sd2e: failed
                      /dev/sd3e: optimal
           No spares.

     In this case there are a number of options. The first option is to add a
     hot spare using:

           # raidctl -a /dev/sd4e raid0

     After the hot add, the status would then be:

                      /dev/sd1e: optimal
                      /dev/sd2e: failed
                      /dev/sd3e: optimal
                      /dev/sd4e: spare

     Reconstruction could then take place using -F as describe above.

     A second option is to rebuild directly onto /dev/sd2e. Once the disk con-
     taining /dev/sd2e has been replaced, one can simply use:

           # raidctl -R /dev/sd2e raid0

     to rebuild the /dev/sd2e component. As the rebuilding is in progress, the
     status will be:

                      /dev/sd1e: optimal
                      /dev/sd2e: reconstructing
                      /dev/sd3e: optimal
           No spares.

     and when completed, will be:

                      /dev/sd1e: optimal
                      /dev/sd2e: optimal
                      /dev/sd3e: optimal
           No spares.

     In circumstances where a particular component is completely unavailable
     after a reboot, a special component name will be used to indicate the
     missing component. For example:

                      /dev/sd2e: optimal
                     component1: failed
           No spares.

     indicates that the second component of this RAID set was not detected at
     all by the auto-configuration code. The name 'component1' can be used
     anywhere a normal component name would be used. For example, to add a hot
     spare to the above set, and rebuild to that hot spare, the following
     could be done:

           # raidctl -a /dev/sd3e raid0
           # raidctl -F component1 raid0

     at which point the data missing from 'component1' would be reconstructed
     onto /dev/sd3e.


     RAID sets can be layered to create more complex and much larger RAID
     sets. A RAID 0 set, for example, could be constructed from four RAID 5
     sets. The following configuration file shows such a setup:

           START array
           # numRow numCol numSpare
           1 4 0

           START disks

           START layout
           # sectPerSU SUsPerParityUnit SUsPerReconUnit RAID_level_0
           128 1 1 0

           START queue
           fifo 100

     A similar configuration file might be used for a RAID 0 set constructed
     from components on RAID 1 sets. In such a configuration, the mirroring
     provides a high degree of redundancy, while the striping provides addi-
     tional speed benefits.

Auto-configuration and Root on RAID

     RAID sets can also be auto-configured at boot. To make a set auto-
     configurable, simply prepare the RAID set as above, and then do a:

           # raidctl -A yes raid0

     to turn on auto-configuration for that set. To turn off auto-
     configuration, use:

           # raidctl -A no raid0

     RAID sets which are auto-configurable will be configured before the root
     filesystem is mounted. These RAID sets are thus available for use as a
     root filesystem, or for any other filesystem. A primary advantage of us-
     ing the auto-configuration is that RAID components become more indepen-
     dent of the disks they reside on. For example, SCSI ID's can change, but
     auto-configured sets will always be configured correctly, even if the
     SCSI ID's of the component disks have become scrambled.

     Having a system's root filesystem (/) on a RAID set is also allowed, with
     the 'a' partition of such a RAID set being used for /. To use raid0a as
     the root filesystem, simply use:

           # raidctl -A root raid0

     To return raid0 to be just an auto-configuring set simply use the -A yes

     Note that kernels can't be directly read from a RAID component. To sup-
     port the root filesystem on RAID sets, some mechanism must be used to get
     a kernel booting. For example, a small partition containing only the
     secondary boot-blocks and an alternate kernel (or two) could be used.
     Once a kernel is booting however, and an auto-configured RAID set is
     found that is eligible to be root, then that RAID set will be auto-
     configured and its 'a' partition (aka raid[0..n]a) will be used as the
     root filesystem. If two or more RAID sets claim to be root devices, then
     the user will be prompted to select the root device. At this time, RAID
     0, 1, 4, and 5 sets are all supported as root devices.

     A typical RAID 1 setup with root on RAID might be as follows:

     1.   wd0a - a small partition, which contains a complete, bootable, basic
          OpenBSD installation.

     2.   wd1a - also contains a complete, bootable, basic OpenBSD installa-

     3.   wd0e and wd1e - a RAID 1 set, raid0, used for the root filesystem.

     4.   wd0f and wd1f - a RAID 1 set, raid1, which will be used only for
          swap space.

     5.   wd0g and wd1g - a RAID 1 set, raid2, used for /usr, /home, or other
          data, if desired.

     6.   wd0h and wd1h - a RAID 1 set, raid3, if desired.

     RAID sets raid0, raid1, and raid2 are all marked as auto-configurable.
     raid0 is marked as being a root-able raid. When new kernels are in-
     stalled, the kernel is not only copied to /, but also to wd0a and wd1a.
     The kernel on wd0a is required, since that is the kernel the system boots
     from. The kernel on wd1a is also required, since that will be the kernel
     used should wd0 fail. The important point here is to have redundant
     copies of the kernel available, in the event that one of the drives fail.

     There is no requirement that the root filesystem be on the same disk as
     the kernel. For example, obtaining the kernel from wd0a, and using sd0e
     and sd1e for raid0, and the root filesystem, is fine. It is critical,
     however, that there be multiple kernels available, in the event of media

     Multi-layered RAID devices (such as a RAID 0 set made up of RAID 1 sets)
     are not supported as root devices or auto-configurable devices at this
     point. (Multi-layered RAID devices are supported in general, however, as
     mentioned earlier.)  Note that in order to enable component auto-
     detection and auto-configuration of RAID devices, the line:

           option    RAID_AUTOCONFIG

     must be in the kernel configuration file. See raid(4) for more details.


     The final operation performed by raidctl is to unconfigure a raid(4) dev-
     ice. This is accomplished via a simple:

           # raidctl -u raid0

     at which point the device is ready to be reconfigured.

Performance Tuning

     Selection of the various parameter values which result in the best per-
     formance can be quite tricky, and often requires a bit of trial-and-error
     to get those values most appropriate for a given system. A whole range of
     factors come into play, including:

     1.   Types of components (e.g. SCSI vs. IDE) and their bandwidth

     2.   Types of controller cards and their bandwidth

     3.   Distribution of components among controllers

     4.   IO bandwidth

     5.   Filesystem access patterns

     6.   CPU speed

     As with most performance tuning, benchmarking under real-life loads may
     be the only way to measure expected performance. Understanding some of
     the underlying technology is also useful in tuning. The goal of this sec-
     tion is to provide pointers to those parameters which may make signifi-
     cant differences in performance.

     For a RAID 1 set, a SectPerSU value of 64 or 128 is typically sufficient.
     Since data in a RAID 1 set is arranged in a linear fashion on each com-
     ponent, selecting an appropriate stripe size is somewhat less critical
     than it is for a RAID 5 set. However: a stripe size that is too small
     will cause large IO's to be broken up into a number of smaller ones,
     hurting performance. At the same time, a large stripe size may cause
     problems with concurrent accesses to stripes, which may also affect per-
     formance. Thus values in the range of 32 to 128 are often the most effec-

     Tuning RAID 5 sets is trickier. In the best case, IO is presented to the
     RAID set one stripe at a time. Since the entire stripe is available at
     the beginning of the IO, the parity of that stripe can be calculated be-
     fore the stripe is written, and then the stripe data and parity can be
     written in parallel. When the amount of data being written is less than a
     full stripe worth, the 'small write' problem occurs. Since a 'small
     write' means only a portion of the stripe on the components is going to
     change, the data (and parity) on the components must be updated slightly
     differently. First, the 'old parity' and 'old data' must be read from the
     components. Then the new parity is constructed, using the new data to be
     written, and the old data and old parity. Finally, the new data and new
     parity are written. All this extra data shuffling results in a serious
     loss of performance, and is typically 2 to 4 times slower than a full
     stripe write (or read). To combat this problem in the real world, it may
     be useful to ensure that stripe sizes are small enough that a 'large IO'
     from the system will use exactly one large stripe write. As is seen
     later, there are some filesystem dependencies which may come into play
     here as well.

     Since the size of a 'large IO' is often (currently) only 32K or 64K, on a
     5-drive RAID 5 set it may be desirable to select a SectPerSU value of 16
     blocks (8K) or 32 blocks (16K). Since there are 4 data sectors per
     stripe, the maximum data per stripe is 64 blocks (32K) or 128 blocks
     (64K). Again, empirical measurement will provide the best indicators of
     which values will yield better performance.

     The parameters used for the filesystem are also critical to good perfor-
     mance. For newfs(8), for example, increasing the block size to 32K or 64K
     may improve performance dramatically. Also, changing the cylinders-per-
     group parameter from 16 to 32 or higher is often not only necessary for
     larger filesystems, but may also have positive performance implications.


     Despite the length of this man-page, configuring a RAID set is a rela-
     tively straight-forward process. All that needs to be done is the follow-
     ing steps:

     1.   Use disklabel(8) to create the components (of type RAID).

     2.   Construct a RAID configuration file: e.g. 'raid0.conf'

     3.   Configure the RAID set with:

                # raidctl -C raid0.conf raid0

     4.   Initialize the component labels with:

                # raidctl -I 123456 raid0

     5.   Initialize other important parts of the set with:

                # raidctl -i raid0

     6.   Get the default label for the RAID set:

                # disklabel raid0 > /tmp/label

     7.   Edit the label:

                # vi /tmp/label

     8.   Put the new label on the RAID set:

                # disklabel -R -r raid0 /tmp/label

     9.   Create the filesystem:

                # newfs /dev/rraid0e

     10. Mount the filesystem:

                # mount /dev/raid0e /mnt

     11. Use:

                # raidctl -c raid0.conf raid0

          to re-configure the RAID set the next time it is needed, or put
          raid0.conf into /etc where it will automatically be started by the
          /etc/rc scripts.


     Certain RAID levels (1, 4, 5, 6, and others) can protect against some
     data loss due to component failure. However the loss of two components of
     a RAID 4 or 5 system, or the loss of a single component of a RAID 0 sys-
     tem will result in the entire filesystem being lost. RAID is NOT a sub-
     stitute for good backup practices.

     Recomputation of parity MUST be performed whenever there is a chance that
     it may have been compromised. This includes after system crashes, or be-
     fore a RAID device has been used for the first time. Failure to keep par-
     ity correct will be catastrophic should a component ever fail -- it is
     better to use RAID 0 and get the additional space and speed, than it is
     to use parity, but not keep the parity correct. At least with RAID 0
     there is no perception of increased data security.


     /dev/{,r}raid*  raid device special files.


     ccd(4), raid(4), rc(8)


     RAIDframe is a framework for rapid prototyping of RAID structures
     developed by the folks at the Parallel Data Laboratory at Carnegie Mellon
     University (CMU). A more complete description of the internals and func-
     tionality of RAIDframe is found in the paper "RAIDframe: A Rapid Proto-
     typing Tool for RAID Systems", by William V. Courtright II, Garth Gibson,
     Mark Holland, LeAnn Neal Reilly, and Jim Zelenka, and published by the
     Parallel Data Laboratory of Carnegie Mellon University.

     The raidctl command first appeared as a program in CMU's RAIDframe v1.1
     distribution. This version of raidctl is a complete re-write, and first
     appeared in NetBSD 1.4 from where it was ported to OpenBSD 2.5.


     Hot-spare removal is currently not available.


     The RAIDframe Copyright is as follows:

     Copyright (c) 1994-1996 Carnegie-Mellon University.
     All rights reserved.

     Permission to use, copy, modify and distribute this software and
     its documentation is hereby granted, provided that both the copyright
     notice and this permission notice appear in all copies of the
     software, derivative works or modified versions, and any portions
     thereof, and that both notices appear in supporting documentation.


     Carnegie Mellon requests users of this software to return to

      Software Distribution Coordinator  or  Software.Distribution@CS.CMU.EDU
      School of Computer Science
      Carnegie Mellon University
      Pittsburgh PA 15213-3890

     any improvements or extensions that they make and grant Carnegie the
     rights to redistribute these changes.

MirBSD #10-current              July 10, 2001                               15

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