Joseph Shraibman wrote:

J. Andrew Rogers wrote:

Do these features make a difference? Far more than you would imagine. On one postgres server I just upgraded, we went from a 3Ware 8x7200-RPM
RAID-10 configuration to an LSI 320-2 SCSI 3x10k RAID-5, with 256M

Is raid 5 much faster than raid 10? On a 4 disk array with 3 data disks and 1 parity disk, you have to write 4/3rds the original data, while on raid 10 you have to write 2 times the original data, so logically raid 5 should be faster.

I think this comparison is a bit simplistic. For example, most raid5 setups have full stripes that are more than 8K (the typical IO size in postgresql), so one might have to read in portions of the stripe in order to compute the parity. The needed bits might be in some disk or controller cache; if it's not then you lose. If one is able to perform full stripe writes then the raid5 config should be faster for writes.

Note also that the mirror has 2 copies of the data, so that the read IOs would be divided across 2 (or more) spindles using round robin or a more advanced algorithm to reduce seek times.

Of course, I might be completely wrong...

-- Alan

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