Re: Postgresql on software RAID

On Tue, 16 Dec 2003, Adam Witney wrote:

>
> Hi all,
>
> I am experimenting with a few OS's for my new hardware. I plan to have a
> software RAID5 device for my pgsql data directory.
>
> I have been experimenting with FreeBSD and with Linux, does anyone have any
> thoughts on whether vinum RAID devices are better than Linux software RAID?
> Or vice versa... Or no difference at all? Any gotchas I should bear in mind?

For a lightly updated database, software RAID5 is a match for hardware
RAID5.  however, as the number of updates / second increase, the hardware
RAID5 can easily outrun the software RAID5 while maintaining data reliably
IF it has a battery backed cache.  This is because the hardware RAID
controller can respond immediately to fsync requests while holding the
actual data in battery backed cache waiting for the right opportunity to
flush said cache, and should a power outage occur, the raid5 hardware
controller will write the cache when the machine powers up.

Linux software RAID seems to have a parallelization issue when layering
RAID 1 or 5 on top of 0 (or vice versa).  I'm not familiar enough with
BSD's RAID layer to comment on it in that light.

They (linux and BSD) seem to be competitive in terms of performance for
straight RAID5 or RAID1 though.