Re: SSDs - SandForce or not?

On 11/14/12 2:11 AM, Toby Corkindale wrote:
> So on the face of it, I think the Sandforce-based drives are probably a
> winner here, so I should look at the Intel 520s for evaluation, and
> whatever the enterprise equivalent are for production.

As far as I know the 520 series drives fail the requirements outlined at
http://wiki.postgresql.org/wiki/Reliable_Writes and you can expect
occasional data corruption after a crash when using them. As such, any
performance results you get back are fake. You can't trust the same
results will come back from their drives that do handle writes
correctly. I'm not aware of any SSD with one of these compressing
Sandforce controller that's on the market right now that does this
correctly; they're all broken for database use. The quick rule of thumb
is that if the manufacturer doesn't brag about the capacitors on the
drive, it doesn't have any and isn't reliable for PostgreSQL.

The safe Intel SSD models state very clearly in the specifications how
they write data in case of a crash. The data sheet for the 320 series
drives for example says "To reduce potential data loss, the Intel® SSD
320 Series also detects and protects from unexpected system power loss
by saving all cached data in the process of being written before
shutting down". The other model I've deployed and know is safe are the
710 series models, which are the same basic drive but with different
quality flash and tuning for longevity. See
http://blog.2ndquadrant.com/intel_ssds_lifetime_and_the_32/ for details.
The 710 series drives are quite a bit more expensive than Intel's
other models.

Intel's recently released DC S3700 drives also look to have the right
battery backup system to be reliable for PostgreSQL. Those are expected
to be significantly cheaper than the 710 models, while having the same
reliability characteristics. I haven't been able to get one yet though,
so I don't really know for sure how well they perform.

--
Greg Smith 2ndQuadrant US greg(at)2ndQuadrant(dot)com Baltimore, MD
PostgreSQL Training, Services, and 24x7 Support www.2ndQuadrant.com