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To: cperciva@FreeBSD.org
From: "M. Warner Losh" <imp@bsdimp.com>
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Cc: cvs-src@FreeBSD.org, src-committers@FreeBSD.org, phk@FreeBSD.org,
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Subject: Re: cvs commit: src/usr.sbin/gstat gstat.c
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In message: <4780CC1B.8090708@freebsd.org>
            Colin Percival <cperciva@FreeBSD.org> writes:
: Poul-Henning Kamp wrote:
: >   When the ms/req fields exceed 1 second, drop the fractions to fit more digits.
: >   
: >   This is unfortunately necessary with some flash based devices which can
: >   get hundreds of seconds behind with softupdates enabled.
: 
: I'm curious, why is this an issue with flash devices?  Given their famous
: ability to perform large numbers of transactions per second, I would have
: expected flash-based drives to be the least likely to have this problem.

Flash is fast for read, but not so fast for write.  Maybe some of the
cool SSD drives solve this problem, but raw flash devices are
relatively slow.  Of course, there's a large range of performance in
even the SD/CF cards in the market today.

Also, our USB stack can cause backlogs when writing to even disks that
are several seconds.  Especially when large numbers of writes hit it.

Warner