Re: best use of an EMC SAN

On 11-Jul-07, at 2:35 PM, Greg Smith wrote:

> On Wed, 11 Jul 2007, Jim Nasby wrote:
>
>> I suppose an entirely in-memory database might be able to swamp a
>> 2 drive WAL as well.
>
> You can really generate a whole lot of WAL volume on an EMC SAN if
> you're doing UPDATEs fast enough on data that is mostly in-memory.
> Takes a fairly specific type of application to do that though, and
> whether you'll ever find it outside of a benchmark is hard to say.
>
Well, this is such an application. The db fits entirely in memory,
and the site is doing over 12M page views a day (I'm not exactly sure
what this translates to in transactions) .
> The main thing I would add as a consideration here is that you can
> configure PostgreSQL to write WAL data using the O_DIRECT path,
> bypassing the OS buffer cache, and greatly improve performance into
> SAN-grade hardware like this.  That can be a big win if you're
> doing writes that dirty lots of WAL, and the benefit is
> straightforward to measure if the WAL is a dedicated section of
> disk (just change the wal_sync_method and do benchmarks with each
> setting).  If the WAL is just another section on an array, how well
> those synchronous writes will mesh with the rest of the activity on
> the system is not as straightforward to predict.  Having the WAL
> split out provides a logical separation that makes figuring all
> this out easier.
>
> Just to throw out a slightly different spin on the suggestions
> going by here:  consider keeping the WAL separate, starting as a
> RAID-1 volume, but keep 2 disks in reserve so that you could easily
> upgrade to a 0+1 set if you end up discovering you need to double
> the write bandwidth.  Since there's never much actual data on the
> WAL disks that would a fairly short downtime operation.  If you
> don't reach a wall, the extra drives might serve as spares to help
> mitigate concerns about the WAL drives burning out faster than
> average because of their high write volume.
>
> --
> * Greg Smith gsmith@gregsmith.com http://www.gregsmith.com
> Baltimore, MD
>
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