Creating a RAID5 Volume

A RAID5 volume uses storage capacity equivalent to one slice in the volume to store redundant information about user data stored on the remainder of the RAID5 volume's slices. The redundant information is distributed across all slices in the volume. Like a mirror, a RAID5 volume increases data availability, but with a minimum of cost in terms of hardware.

The system must contain at least three state database replicas before you can create RAID5 volumes.

A RAID5 volume can only handle a single slice failure.

Follow the 20-percent rule when creating a RAID5 volume: because of the complexity of parity calculations, volumes with greater than about 20 percent writes should probably not be RAID5 volumes. If data redundancy is needed, consider mirroring.

There are drawbacks to a slice-heavy RAID5 volume: the more slices a RAID5 volume contains, the longer read and write operations will take if a slice fails.

A RAID5 volume must consist of at least three slices. A RAID5 volume can be grown by concatenating additional slices to the volume. The new slices do not store parity information, however they are parity protected. The resulting RAID5 volume continues to handle a single slice failure.

The interlace value is key to RAID5 performance. It is configurable at the time the volume is created; thereafter, the value cannot be modified. The default interlace value is 16 Kbytes. This is reasonable for most applications.

Use the same size disk slices. Creating a RAID5 volume from different size slices results in unused disk space in the volume.

Do not create a RAID5 volume from a slice that contains an existing file system. Doing so will erase the data during the RAID5 initialization process.

RAID5 volumes cannot be striped, concatenated, or mirrored.

1. The following example creates a RAID 5 volume using 3 slices that will be named /dev/md/rdsk/d3 with the metainit command. Of the twelve disks available in the D1000 Disk Array, I will be using slices c1t4d0s7, c2t4d0s7, and c1t5d0s7 as follows:

   # metainit d3 -r c1t4d0s7 c2t4d0s7 c1t5d0s7
   d3: RAID is setup

   Let's explain the details of the above example. The RAID5 volume d3 is created with the -r option from three slices. Because no interlace is specified, d3 uses the default of 16 Kbytes. The system verifies that the RAID5 volume has been set up, and begins initializing the volume.

    

2. Use the metastat command to query your new RAID5 volumes. After running the above command, the volume will go through an initialization state. This may take several minutes to complete. When using the metastat command, you will be able to view how far of the initialization is completed. You must wait for the initialization to finish before you can use the new RAID5 volume. The following screenshot shows the RAID5 volume during its initialization phase:

   # metastat d3
   d3: RAID
       State: Initializing
       Initialization in progress: 32.0% done
       Interlace: 32 blocks
       Size: 35331849 blocks (16 GB)
   Original device:
       Size: 35334720 blocks (16 GB)
           Device     Start Block  Dbase        State Reloc  Hot Spare
           c1t4d0s7      11103       Yes Initializing   Yes
           c2t4d0s7      11103       Yes Initializing   Yes
           c1t5d0s7      11103       Yes Initializing   Yes
   
   Device Relocation Information:
   Device   Reloc  Device ID
   c1t4d0   Yes    id1,sd@SSEAGATE_ST39102LCSUN9.0GLJP248260000194511NU
   c2t4d0   Yes    id1,sd@SSEAGATE_ST39102LCSUN9.0GLJP1841500002945H5FE
   c1t5d0   Yes    id1,sd@SSEAGATE_ST39102LCSUN9.0GLJE34597000029290C8N

   When the disks within the RAID5 volume are completed with their initialization phase, this is what it will look like:

   # metastat d3
   d3: RAID
       State: Okay
       Interlace: 32 blocks
       Size: 35331849 blocks (16 GB)
   Original device:
       Size: 35334720 blocks (16 GB)
           Device     Start Block  Dbase        State Reloc  Hot Spare
           c1t4d0s7      11103       Yes         Okay   Yes
           c2t4d0s7      11103       Yes         Okay   Yes
           c1t5d0s7      11103       Yes         Okay   Yes
   
   Device Relocation Information:
   Device   Reloc  Device ID
   c1t4d0   Yes    id1,sd@SSEAGATE_ST39102LCSUN9.0GLJP248260000194511NU
   c2t4d0   Yes    id1,sd@SSEAGATE_ST39102LCSUN9.0GLJP1841500002945H5FE
   c1t5d0   Yes    id1,sd@SSEAGATE_ST39102LCSUN9.0GLJE34597000029290C8N

    

3. Now that we have created our RAID5 volume, we can now pretend that the volume is a big partition (slice) on which we can do the usual file system things. Let's now create a UFS file system using the newfs command. I want to create a UFS file system with an 8KB block size:

   # newfs -i 8192 /dev/md/rdsk/d3
   newfs: construct a new file system /dev/md/rdsk/d3: (y/n)? y
   Warning: 1 sector(s) in last cylinder unallocated
   /dev/md/rdsk/d3:        35331848 sectors in 9839 cylinders of 27 tracks, 133 sectors
           17251.9MB in 615 cyl groups (16 c/g, 28.05MB/g, 3392 i/g)
   super-block backups (for fsck -F ufs -o b=#) at:
    32, 57632, 115232, 172832, 230432, 288032, 345632, 403232, 460832, 518432,
   Initializing cylinder groups:
   ............
   super-block backups for last 10 cylinder groups at:
    34765088, 34822688, 34880288, 34933280, 34990880, 35048480, 35106080,
    35163680, 35221280, 35278880,

    

4. Finally, we mount the file system on /db3 as follows:

   # mkdir /db3
   # mount -F ufs /dev/md/dsk/d3 /db3

    

5. To ensure that this new file system is mounted each time the machine is started, insert the following line into you /etc/vfstab file (all on one line with tabs separating the fields):

   /dev/md/dsk/d3       /dev/md/rdsk/d3      /db3  ufs     2       yes     -

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Last modified: March 12, 2019