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