Re: SSD Drives - Mailing list pgsql-general

On 04/04/2014 10:15 AM, Merlin Moncure wrote:
>> 2. Do I need both BBU on the RAID *and* capacitor on the SSD or just on one?
>> Which one? I'm suspecting capacitor on the SSD and write-through on the
>> RAID.
> You need both. The capacitor protects the drive, the BBU protects the
> raid controller.
?? In write-through the controller shouldn't return success until it
gets it from the drive so no BBU should be required. One LSI slide deck
recommends write-back as the optimum policy for SSDs. But I could be
wrong which is why I ask.
>> 2. Current thoughts on hardware vs. software RAID - especially since many of
>> the current SSD solutions plug straight into the bus.
> IMNSHO, software raid is a better bet.  The advantages are compelling:
> Cost, TRIM support, etc. and the SSD drives do not benefit as much
> from the write cache.   But hardware controllers offer very fast burst
> write performance which is nice.
>
> 6. Thoughts on "best bang for the buck?" For example, am I better off
> dropping the RAID cards and additional drives and instead adding another
> standby server?
> This is going to depend a lot on write patterns.  If you don't do much
> writing, you can gear up accordingly.  For all around performance, the
> S3700 (2.5$/gb) IMO held the crown for most of 2013 and I think is
> still the one to buy.  The s3500 (1.25$/gb) came out and also looks
> like a pretty good deal, and there are some decent competitors (600
> pro for example).  If you're willing to spend more, there are a lot of
> other options.  I don't think it's reasonable to spend less for a
> write heavy application.

FWIW, the workload is somewhat over 50% writes and currently peaks at
~1,600 queries/second after excluding "set" statements. This is
currently spread across four 15k SATA drives in RAID 10.

Judicious archiving allows us to keep our total OS+data storage
requirements under 100GB. Usually. So we should be able to easily stay
in the $500/drive price range (200GB S3700) and still have plenty of
headroom for wear-leveling.

One option I'm considering is no RAID at all but spend the savings from
the controllers and extra drives toward an additional standby server.

Cheers,
Steve