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Although both mirroring (RAID-1) and RAID-5 provide redundancy of data, they use different methods. Mirroring provides data redundancy by maintaining multiple complete copies of the data in a volume. Data being written to a mirrored volume is reflected in all copies. If a portion of a mirrored volume fails, the system continues to use the other copies of the data.
RAID-5 provides data redundancy by using parity. Parity is a calculated value used to reconstruct data after a failure. While data is being written to a RAID-5 volume, parity is calculated by doing an exclusive OR (XOR) procedure on the data. The resulting parity is then written to the volume. The data and calculated parity are contained in a plex that is "striped" across multiple disks. If a portion of a RAID-5 volume fails, the data that was on that portion of the failed volume can be recreated from the remaining data and parity information. It is also possible to mix concatenation and striping in the layout.
Parity locations in a RAID-5 mode shows parity locations in a RAID-5 array configuration.
Parity locations in a RAID-5 mode
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Every stripe has a column containing a parity stripe unit and columns containing data. The parity is spread over all of the disks in the array, reducing the write time for large independent writes because the writes do not have to wait until a single parity disk can accept the data.
RAID-5 volumes can additionally perform logging to minimize recovery time. RAID-5 volumes use RAID-5 logs to keep a copy of the data and parity currently being written. RAID-5 logging is optional and can be created along with RAID-5 volumes or added later.
See "Veritas Volume Manager RAID-5 arrays" on page 49.
Note
VxVM supports RAID-5 for private disk groups, but not for shareable disk groups in a cluster environment. In addition, VxVM does not support the mirroring of RAID-5 volumes that are configured using Veritas Volume Manager software. Disk devices that support RAID-5 in hardware may be mirrored.
A traditional RAID-5 array is several disks organized in rows and columns. A column is a number of disks located in the same ordinal position in the array. A row is the minimal number of disks necessary to support the full width of a parity stripe.
Traditional RAID-5 array shows the row and column arrangement of a traditional RAID-5 array.
Traditional RAID-5 array
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This traditional array structure supports growth by adding more rows per column. Striping is accomplished by applying the first stripe across the disks in Row 0, then the second stripe across the disks in Row 1, then the third stripe across the Row 0 disks, and so on. This type of array requires all disks columns, and rows to be of equal size.
The RAID-5 array structure in Veritas Volume Manager differs from the traditional structure. Due to the virtual nature of its disks and other objects, VxVM does not use rows.
Veritas Volume Manager RAID-5 array shows how VxVM uses columns consisting of variable length subdisks, where each subdisk represents a specific area of a disk.
Veritas Volume Manager RAID-5 array
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VxVM allows each column of a RAID-5 plex to consist of a different number of subdisks. The subdisks in a given column can be derived from different physical disks. Additional subdisks can be added to the columns as necessary. Striping is implemented by applying the first stripe across each subdisk at the top of each column, then applying another stripe below that, and so on for the length of the columns. Equal-sized stripe units are used for each column. For RAID-5, the default stripe unit size is 16 kilobytes.
See "Striping (RAID-0)" on page 40.
Note
Mirroring of RAID-5 volumes is not supported.
See "Creating a RAID-5 volume" on page 266.
There are several layouts for data and parity that can be used in the setup of a RAID-5 array. The implementation of RAID-5 in VxVM uses a left-symmetric layout. This provides optimal performance for both random I/O operations and large sequential I/O operations. However, the layout selection is not as critical for performance as are the number of columns and the stripe unit size.
Left-symmetric layout stripes both data and parity across columns, placing the parity in a different column for every stripe of data. The first parity stripe unit is located in the rightmost column of the first stripe. Each successive parity stripe unit is located in the next stripe, shifted left one column from the previous parity stripe unit location. If there are more stripes than columns, the parity stripe unit placement begins in the rightmost column again.
Left-symmetric layout shows a left-symmetric parity layout with five disks (one per column).
Left-symmetric layout
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For each stripe, data is organized starting to the right of the parity stripe unit. In the figure, data organization for the first stripe begins at P0 and continues to stripe units 0-3. Data organization for the second stripe begins at P1, then continues to stripe unit 4, and on to stripe units 5-7. Data organization proceeds in this manner for the remaining stripes.
Each parity stripe unit contains the result of an exclusive OR (XOR) operation performed on the data in the data stripe units within the same stripe. If one column's data is inaccessible due to hardware or software failure, the data for each stripe can be restored by XORing the contents of the remaining columns data stripe units against their respective parity stripe units.
For example, if a disk corresponding to the whole or part of the far left column fails, the volume is placed in a degraded mode. While in degraded mode, the data from the failed column can be recreated by XORing stripe units 1-3 against parity stripe unit P0 to recreate stripe unit 0, then XORing stripe units 4, 6, and 7 against parity stripe unit P1 to recreate stripe unit 5, and so on.
Failure of more than one column in a RAID-5 plex detaches the volume. The volume is no longer allowed to satisfy read or write requests. Once the failed columns have been recovered, it may be necessary to recover user data from backups.
Logging is used to prevent corruption of data during recovery by immediately recording changes to data and parity to a log area on a persistent device such as a volume on disk or in non-volatile RAM. The new data and parity are then written to the disks.
Without logging, it is possible for data not involved in any active writes to be lost or silently corrupted if both a disk in a RAID-5 volume and the system fail. If this double-failure occurs, there is no way of knowing if the data being written to the data portions of the disks or the parity being written to the parity portions have actually been written. Therefore, the recovery of the corrupted disk may be corrupted itself.
Incomplete write to a RAID-5 volume shows a RAID-5 volume configured across three disks (A, B and C).
Incomplete write to a RAID-5 volume
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In this volume, recovery of disk B's corrupted data depends on disk A's data and disk C's parity both being complete. However, only the data write to disk A is complete. The parity write to disk C is incomplete, which would cause the data on disk B to be reconstructed incorrectly.
This failure can be avoided by logging all data and parity writes before committing them to the array. In this way, the log can be replayed, causing the data and parity updates to be completed before the reconstruction of the failed drive takes place.
Logs are associated with a RAID-5 volume by being attached as log plexes. More than one log plex can exist for each RAID-5 volume, in which case the log areas are mirrored.
See "Adding a RAID-5 log" on page 293.