Raid 5 performance

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RAID 5
RAID configuration
RAID Performance
We're going to re-do the disk configuration on a server due to need for more disk space. It's currently mirrored, raid 1, with 2 10,000rpm drives. We are planning to switch to raid 5 with 4 15,000 drives. Would this be bad for performance?

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As described here, everything depends on what kind of data will be writtend and the average of read/writes.
Hereafter an excerpt:

<b>RAID 5 performance</b>

<i>RAID 5 implementations suffer from poor performance when faced with a workload which includes many writes which are smaller than the capacity of a single stripe;[citation needed] this is because parity must be updated on each write, requiring read-modify-write sequences for both the data block and the parity block. More complex implementations may include a non-volatile write back cache to reduce the performance impact of incremental parity updates.

Random write performance is poor, especially at high concurrency levels common in large multi-user databases. The read-modify-write cycle requirement of RAID 5′s parity implementation penalizes random writes by as much as an order of magnitude compared to RAID 0.[12]

Performance problems can be so severe that some database experts have formed a group called BAARF — the Battle Against Any Raid Five.[13]

The read performance of RAID 5 is almost as good as RAID 0 for the same number of disks. Except for the parity blocks, the distribution of data over the drives follows the same pattern as RAID 0. The reason RAID 5 is slightly slower is that the disks must skip over the parity blocks.

In the event of a system failure while there are active writes, the parity of a stripe may become inconsistent with the data. If this is not detected and repaired before a disk or block fails, data loss may ensue as incorrect parity will be used to reconstruct the missing block in that stripe. This potential vulnerability is sometimes known as the write hole. Battery-backed cache and similar techniques are commonly used to reduce the window of opportunity for this to occur</i>

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