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Table 4-2 Disk 1 Bit 1 0
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An Example of RAID Parity
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Disk 2 Bit 1 1 Disk 3 Bit 1 1 Disk 4 Bit 1 1 Disk 5 Parity Bit 1 Sum of Bits 4 (even)
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Think of the parity as being created on single bits. Even though the disk stripe contains many bits, you make the data recoverable by creating a parity on the single bits. As you can see from Table 4-2, the parity is actually created on individual bits in the stripes. Even though the disk drives are broken up into chunks or stripe pieces that might be 64 KB or larger, the parity can be created only at the bit level, as shown here. Parity is actually calculated with a more sophisticated algorithm than that just described. For example, suppose Disk 3 fails. In this case, the parity bit plus the bits from the other disk drives can be used to recover the missing bit from Disk 3 because they must all add up to an even number.
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Creating the Parity As you have seen in this section, the RAID-5 parity is created by finding the sum of the same bits on all of the drives in the RAID-5 array and then creating a parity bit so that the result is even. Well, as you might imagine, it is impractical for an array controller to read all of the data from all of the drives each time an I/O operation occurs. This would be inefficient and slow.
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When a RAID-5 array is created, the data is initially zeroed out, and the parity bit is created. You then have a set of RAID-5 disk drives with no data but with a full set of parity bits. From this point on, whenever data is written to a disk drive, both the data disk and the parity disk must be read, then the new data is compared with the old data, and if the data for a particular bit has changed, the parity for that bit must be changed. This is accomplished with an exclusive OR (XOR) operation. Thus, only the data disk and the parity
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I/O Subsystem Planning and RAID Configuration
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disk, not all of the disks in the array, need to be read. Once this operation has been completed, both disk drives must be written out because the parity operation works on entire stripes. Therefore, for each write to a RAID-5 volume, four physical I/O operations occur: two reads (one from data and one from parity) and two writes (back to data and back to parity). However, with a RAID-5 array, the parity is distributed, so this load should be balanced among all the disk drives in the array. This process consists of the following steps: 1. Write to the RAID-5 array occurs. 2. Current data is read from the disk. 3. Current parity is read from the disk. 4. Exclusive Or (XOR) operation is performed on data and new parity is calculated. 5. Data is written out to the disk. 6. Parity is written out to the disk. As you can see, RAID-5 incurs significant overhead on writes. However, reads do not incur any additional overhead (unless a drive has failed). RAID-5 Recommendations Because of the additional I/O operations incurred by RAID-5 writes, this RAID level is recommended for disk volumes that are used mostly for reading. Because the parity is distributed among the various disks in the array, all disks are used for read operations. Because of this characteristic, the following suggestions are recommended:
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Use RAID-5 on read-only volumes Any disk volume that does more than 10 percent writes is not a good candidate for RAID-5. Use write caching on RAID-5 volumes Because a RAID-5 write will not be completed until two reads and two writes have occurred, the response time of writes can be improved through the use of a write cache. (When using a write cache, be sure that it is backed up by battery power.) However, the write cache is not a cure for overdriving your disk drives. You must still stay within the capacity of those disks.
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As you can see, RAID-5 is economical, but you pay a performance price. You will see later in this chapter how high that price can be.
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