Re: raid writethrough mode (WT), ssds and your DB. (was Performances issues with SSD volume ?)

On Thu, May 21, 2015 at 11:21:49AM +0000, Graeme B. Bell wrote:
> > Not using your raid controllers write cache then? Not sure just how
> > important that is with SSDs these days, but if you've got a BBU set
> > it to "WriteBack". Also change "Cache if Bad BBU" to "No Write Cache
> > if Bad BBU" if you do that.
>
> I did quite a few tests with WB and WT last year.
>
> - WT should be OK with e.g. Intel SSDs. From memory I saw write
> performance gains of about 20-30% with Crucial M500/M550 writes on a
> Dell H710 RAID controller. BUT that controller didn't have WT fastpath
> though which is absolutely essential to see substantial gains WT. I
> expect with WT and a fastpath enabled RAID you'd see much higher
> numbers, e.g. 100%+ higher IOPS.
>
> (So, if you don't have fastpath on your controller, you might as
> well plan to leave WB on and just buy cheaper SSD drives rather than
> expensive ones - the raid controller will be your choke point for
> performance on WT and it's a source of risk).
>
> - WT with most SSDs will likely corrupt your postgres database the
> first time you lose power. (on all the drives I've tested)
>
> - WB is the only safe option unless you have done lots of plug pull
> tests on a drive that is guaranteed to protect data "in flight" during
> power loss (Intel disks + maybe the new samsung pcie).

I think you have WT (write-through) and WB (write-back) reversed above.

--
Bruce Momjian <bruce(at)momjian(dot)us> http://momjian.us
EnterpriseDB http://enterprisedb.com

+ Everyone has their own god. +