Re: SSDs with Postgresql?

On 04/20/2011 01:50 AM, Toby Corkindale wrote:
> Also, the number of erase cycles you can get, over the whole disk, is 
> quite large on modern disks!
>
> So large that you'll probably go decades before you wear the disk out, 
> even with continual writes.
>
> Don't buy into the SSD FUD myths..

There is no FUD being spread here.  Particularly given the PostgreSQL 
WAL write pattern, it's not impossible to wear out a SSD placed there in 
a small number of years.  A system with a trivial but not completely 
idle workload will generate one 16MB WAL segment every 5 minutes, which 
works out to 4.5GB/day of writes.  That's the baseline--the reality is 
much, much higher than that on most systems.  The fact that every row 
update can temporarily use more than 8K means that actual write 
throughput on the WAL can be shockingly large.  The smallest customer I 
work with regularly has a 50GB database, yet they write 20GB of WAL 
every day.  You can imagine how much WAL is generated daily on systems 
with terabyte databases.

As for what this translates into in the real world, go read 
http://archives.postgresql.org/message-id/BANLkTi=GsyBfq+ApWPR_qCA7AN+NqT=zww@mail.gmail.com 
as one worked out sample.  Anyone deploying PostgreSQL onto MLC can't 
necessarily ignore this issue.

-- 
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