On Sun, 26 Oct 2003, Mark Kirkwood wrote:
> Got to going this today, after a small delay due to the arrival of new
> disks,
>
> So the system is 2x700Mhz PIII, 512 Mb, Promise TX2000, 2x40G ATA-133
> Maxtor Diamond+8 .
> The relevent software is Freebsd 4.8 and Postgresql 7.4 Beta 2.
>
> Two runs of 'pgbench -c 50 -t 1000000 -s 10 bench' with a power cord
> removal after about 2 minutes were performed, one with hw.ata.wc = 1
> (write cache enabled) and other with hw.ata.wc = 0 (disabled).
>
> In *both* cases the Pg server survived - i.e it came up, performed
> automatic recovery. Subsequent 'vacuum full' and further runs of pgbench
> completed with no issues.
Sweet. It may be that the promise is turning off the cache, or that the
new generation of IDE drives is finally reporting fsync correctly. Was
there a performance difference in the set with write cache on or off?
> I would conclude that it not *always* the case that power failure
> renders the database unuseable.
But it usually is if write cache is enabled.
> I have just noticed a similar posting from Scott were he finds the cache
> enabled case has an dead database after power failure. It seems that
> it's a question of how *likely* is it that the database will survive/not
> survive a power failure...
>
> The other interesting possibility is that Freebsd with soft updates
> helped things remain salvageable in the cache enabled case (as some
> writes *must* be lost at power off in this case)....
Free BSD may be the reason here. If it's softupdates are ordered in the
right way, it may be that even with write caching on, the drives "do the
right thing" under BSD. Time to get out my 5.0 disks and start playing
with my test server. Thanks for the test!
