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Re: Two hard drives --- what to do with them?

From: "Peter Kovacs" <maxottovonstirlitz(at)gmail(dot)com>
To: "Shane Ambler" <pgsql(at)sheeky(dot)biz>
Cc: "Jeff Davis" <pgsql(at)j-davis(dot)com>, "Tom Lane" <tgl(at)sss(dot)pgh(dot)pa(dot)us>, "Carlos Moreno" <moreno_pg(at)mochima(dot)com>, pgsql-performance(at)postgresql(dot)org
Subject: Re: Two hard drives --- what to do with them?
Date: 2007-02-27 08:27:52
Message-ID: (view raw, whole thread or download thread mbox)
Lists: pgsql-performance
On 2/27/07, Shane Ambler <pgsql(at)sheeky(dot)biz> wrote:
> Jeff Davis wrote:
> >> Sorry for for not being familar with storage techonologies... Does
> >> "battery" here mean battery in the common sense of the word - some
> >> kind of independent power supply? Shouldn't the disk itself be backed
> >> by a battery? As should the entire storage subsystem?
> >>
> >
> > Yes, a battery that can hold power to keep data alive in the write cache
> > in case of power failure, etc., for a long enough time to recover and
> > commit the data to disk.
> Just to expand a bit - the battery backup options are available on some
> raid cards - that is where you would be looking for it. I don't know of
> any hard drives that have it built in.
> Of cause another reason to have a UPS for the server - keep it running
> long enough after the clients have gone down so that it can ensure
> everything is on disk and shuts down properly.
> > So, a write cache is OK (even for pg_xlog) if it is durable (i.e. on
> > permanent storage or backed by enough power to make sure it gets there).
> > However, if PostgreSQL has no way to know whether a write is durable or
> > not, it can't guarantee the data is safe.
> >
> > The reason this becomes an issue is that many consumer-grade disks have
> > write cache enabled by default and no way to make sure the cached data
> > actually gets written. So, essentially, these disks "lie" and say they
> > wrote the data, when in reality, it's in volatile memory. It's
> > recommended that you disable write cache on such a device.
>  From all that I have heard this is another advantage of SCSI disks -
> they honor these settings as you would expect - many IDE/SATA disks
> often say "sure I'll disable the cache" but continue to use it or don't
> retain the setting after restart.

As far as I know, SCSI drives also have "write cache" which is turned
off by default, but can be turned on (e.g. with the sdparm utility on
Linux). The reason I am so much interested in how write cache is
typically used (on or off) is that I recently ran our benchmarks on a
machine with SCSI disks and those benchmarks with high commit ratio
suffered significantly compared to our previous results
"traditionally" obtained on machines with IDE drives.

I wonder if running a machine on a UPS + 1 hot standby internal PS is
equivalent, in terms of data integrity, to using battery backed write
cache. Instinctively, I'd think that UPS + 1 hot standby internal PS
is better, since this setup also provides for the disk to actually
write out the content of the cache -- as you pointed out.


> --
> Shane Ambler
> pgSQL(at)Sheeky(dot)Biz
> Get Sheeky @ http://Sheeky.Biz

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