Re: SSD + RAID

On Tue, Nov 17, 2009 at 9:54 AM, Brad Nicholson
<bnichols(at)ca(dot)afilias(dot)info> wrote:
> On Tue, 2009-11-17 at 11:36 -0500, Merlin Moncure wrote:
>> 2009/11/13 Greg Smith <greg(at)2ndquadrant(dot)com>:
>> > As far as what real-world apps have that profile, I like SSDs for small to
>> > medium web applications that have to be responsive, where the user shows up
>> > and wants their randomly distributed and uncached data with minimal latency.
>> > SSDs can also be used effectively as second-tier targeted storage for things
>> > that have a performance-critical but small and random bit as part of a
>> > larger design that doesn't have those characteristics; putting indexes on
>> > SSD can work out well for example (and there the write durability stuff
>> > isn't quite as critical, as you can always drop an index and rebuild if it
>> > gets corrupted).
>>
>> I am right now talking to someone on postgresql irc who is measuring
>> 15k iops from x25-e and no data loss following power plug test.  I am
>> becoming increasingly suspicious that peter's results are not
>> representative: given that 90% of bonnie++ seeks are read only, the
>> math doesn't add up, and they contradict broadly published tests on
>> the internet.  Has anybody independently verified the results?
>
> How many times have the run the plug test?  I've read other reports of
> people (not on Postgres) losing data on this drive with the write cache
> on.

When I run the plug test it's on a pgbench that's as big as possible
(~4000) and I remove memory if there's a lot in the server so the
memory is smaller than the db. I run 100+ concurrent and I set
checkoint timeouts to 30 minutes, and make a lots of checkpoint
segments (100 or so), and set completion target to 0. Then after
about 1/2 checkpoint timeout has passed, I issue a checkpoint from the
command line, take a deep breath and pull the cord.