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Thread: Raid Controller Write Cache setting for WAL and Data

Raid Controller Write Cache setting for WAL and Data

From
Piotr Gasidło
Date:
Hello,

Is it safe, to have RAID controller (not BBC) write cache _enabled_
for disks where data are stored and write cache _disabled_ for disks
where WAL segments are stored?
I _can_ afford of loosing some data in case of power failure. But I'm
afraid of having database in unrecoverable state after crash.

fsync = on

I have 4 disks, 2 for WAL (RAID1) and 2 for data (RAID1). WAL and data
are on XFS partitions (nobarrier, noatime).

--
Piotr Gasidło

Re: Raid Controller Write Cache setting for WAL and Data

From
Andrew Sullivan
Date:
On Wed, Feb 16, 2011 at 04:40:43PM +0100, Piotr Gasidło wrote:

> Is it safe, to have RAID controller (not BBC) write cache _enabled_
> for disks where data are stored and write cache _disabled_ for disks
> where WAL segments are stored?

No.

> I _can_ afford of loosing some data in case of power failure. But I'm
> afraid of having database in unrecoverable state after crash.

It could be.  Suppose you had the crash at the exact moment when the
the WAL was getting flushed.  Now you'll have a corrupt table.  It
will likely start up, but some time later when someone goes to get
that data, you'll run into a problem.

A battery is one of the simplest and cheapest things you can do to
make your database system more reliable and faster at the same time.

A


--
Andrew Sullivan
ajs@crankycanuck.ca

Re: Raid Controller Write Cache setting for WAL and Data

From
Greg Smith
Date:
Piotr Gasidło wrote:
> I _can_ afford of loosing some data in case of power failure. But I'm
> afraid of having database in unrecoverable state after crash.
>

Then turn off synchronous_commit.  That's exactly the behavior you get
when it's disabled:  some data loss after a crash, no risk of database
corruption, and faster performance without needing a controller with a
battery.

If you've already got a RAID controller that accepts a battery, it would
be silly not to then buy one though.  The controller is normally 75% of
the price of the combination, so getting that but not the final piece to
really make it perform well wouldn't be a good move.

--
Greg Smith   2ndQuadrant US    greg@2ndQuadrant.com   Baltimore, MD
PostgreSQL Training, Services, and 24x7 Support  www.2ndQuadrant.us
"PostgreSQL 9.0 High Performance": http://www.2ndQuadrant.com/books