Postgres Pro
Downloads
Home   >   mailing lists

Thread: Options for fsync?

Options for fsync?

From
A B
Date:
Hi there!

I'm trying to comprehend the current status of filesystem settings and
PostgreSQL settings.

If I run on a machine (using Linux and ext3) with no battery backed
raid controller then I should use fsync=on and disable the write cache
on my harddrives  to avoid corruption.

If I had a controller with battery, then I could use fsync=off, but I
should still disable the write cache?

Re: Options for fsync?

From
John R Pierce
Date:
A B wrote:
> Hi there!
>
> I'm trying to comprehend the current status of filesystem settings and
> PostgreSQL settings.
>
> If I run on a machine (using Linux and ext3) with no battery backed
> raid controller then I should use fsync=on and disable the write cache
> on my harddrives  to avoid corruption.
>
> If I had a controller with battery, then I could use fsync=off, but I
> should still disable the write cache?
>


no, you would still want fsync on, otherwise commits are left in main
memory.     fsync forces the commits to be written to the controller
cache which has the battery.    most of the raid systems I've used that
battery backed cache have storage media with proper cache flushing, and
the raid controller issues the appropriate commands to insure the
on-drive buffers get flushed too.  Its usually when you use cheap
desktop SATA that you can get into trouble, as long as you're using
proper server grade storage like SAS, FC, SCSI, it generally works right.

Re: Options for fsync?

From
Greg Smith
Date:
A B wrote:
> If I had a controller with battery, then I could use fsync=off, but I
> should still disable the write cache?
>

Never turn fsync=off.  Always disable any write cache on the drives
themselves; most good battery-backed RAID controllers will do that for
you.  So long as the battery continues to work, you can keep the write
cache on the controller itself enabled, and in fact you have to do so in
order to get good commit performance.  Given that eventually all
batteries fail, the better cards on the market actually make that an
explicit setting, which is the safest setting if available:  write-back
cache enabled when battery works, write-through [cache disabled] if doesn't.

Make sure you setup monitoring alerts via e-mail for drive and battery
failures too; test them out by yanking at drive before the server goes
into production if you can.

--
Greg Smith  2ndQuadrant US  Baltimore, MD
PostgreSQL Training, Services and Support
greg@2ndQuadrant.com   www.2ndQuadrant.us