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Comment by amitutk | Hacker News Reader

noob q: any reason not to use RAID5 or 6?

As the others mention, read performance is bad while in a degraded state. However, seeing as they're running their log on the same drives, they have quite a lot of write ops as well.

RAID5 is notoriously bad for write performance, just as RAID6 is. This is a classic writeup on why you should avoid RAID5 at all costs for a scenario like this: http://www.miracleas.com/BAARF/RAID5_versus_RAID10.txt

Good question. I just googled this: http://en.wikipedia.org/wiki/RAID#RAID_10_versus_RAID_5_in_R...

Summary: Slight benefit to read performance and greater resiliency in the case of drive failure with Raid 10.

RAID-5 and RAID-6 suffer tremendously in their degraded state - instead of being able to read individual blocks from a single disk, you need to read all the disks (or all by 1 in RAID-6) for any blocks stored on the down drive.

If you run RAID 10 on a busy system and a disk fails, everything keeps running at full speed until you replace the missing disk and rebuild your redundancy.

If you run RAID 5 or 6 and a disk fails, suddenly every read operation becomes several times slower because the missing data must be computed from the parity and remaining data channels. If your normal day-to-day load on the storage is too high you are screwed until you rebuild your missing disk.

Write throughtput/latency on RAID10 remains consistent with previous numbers (latency perhaps milliseconds faster), but read latency and overall-throughput (as opposed to individual requests) will suffer on the mirror with the bad disk.

Recovery will drop read/write performance, depending on how fast you do it, but nowhere near as bad as RAID-5/6.

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