Re: Plan: journalling fixes for WAPBL

[David Holland <dholland-tech%netbsd.org@localhost>]

(from a while back)

On Wed, Sep 28, 2016 at 02:27:39PM +0000, Paul.Koning%dell.com@localhost wrote:
 > > On Sep 28, 2016, at 7:22 AM, Jarom?r Dole?ek <jaromir.dolecek%gmail.com@localhost> wrote:
 > > I think it's far assesment to say that on SATA with NCQ/31 tags (max
 > > is actually 31, not 32 tags), it's pretty much impossible to have
 > > acceptable write performance without using write cache. We could never
 > > saturate even drive with 16MB cache with just 31 tags and 64k maxphys.
 > > So it's IMO not useful to design for world without disk drive write
 > > cache.
 > 
 > I think that depends on the software.  In a SAN storage array I
 > work on, we used to use SATA drives, always with cache disabled to
 > avoid data loss due to power failure.  We had them running just
 > fine with NCQ.  (For that matter, even without NCQ, though that
 > takes major effort.)

Well, there's two things going on here. One is the parallelism limit,
which you can work around by having more drives, e.g. in an array.
The other is netbsd's 64k MAXPHYS issue, which is our own problem that
we could put time into. (And in fact, it would be nice to get
tls-maxphys into -8... anyone have time? I don't. sigh)

However, I'm missing something. The I/O queue depths that you need to
get peak write performance from SSDs are larger than 31, and the test
labs appear to have been able to do this with SATA-attached SSDs...
what are/were they doing?

-- 
David A. Holland
dholland%netbsd.org@localhost