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Date: Wed, 31 Aug 2011 12:11:02 +0400
From: Lev Serebryakov <lev@FreeBSD.org>
Organization: FreeBSD
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To: Kirk McKusick <mckusick@mckusick.com>
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References: <1693072185.20110831022933@serebryakov.spb.ru>
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Cc: freebsd-fs@FreeBSD.org, lev@FreeBSD.org
Subject: Re: Very inconsistent (read) speed on UFS2
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Hello, Kirk.
You wrote 31 =E0=E2=E3=F3=F1=F2=E0 2011 =E3., 3:00:45:

> So, my new theory on why your O_DIRECT test is running slowly is due
> to the single threading in the GEOM layer. Pawel Jakub Dawidek (pjd@)
> gave a very interesting talk on this problem at this year's BSDCan.
  I want to stress my point: it is not low speed, what especially
 bother me. It is inconsistency.

  Bad drive, always-single-threaded-GEOM, etc., should give consistent
 slow speed.

  Bad code in geom_raid5 could give inconsistent write speed, du to
caching, but reading path is as straight and simple as possible here.

  Bad drive or AST-GEOM is not what simple to fix (ok, bad drive IS
simple to replace, of course). But I suspect, that here is some simple
"misunderstanding" between geom_raid5 code and VFS/FFS2 layer, which I
could fix. But I don't know where to look at.

--=20
// Black Lion AKA Lev Serebryakov <lev@FreeBSD.org>