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Date: Sun, 6 Oct 2002 23:42:01 -0700 (PDT)
From: Matthew Dillon <dillon@apollo.backplane.com>
Message-Id: <200210070642.g976g1nO045529@apollo.backplane.com>
To: "Greg 'groggy' Lehey" <grog@FreeBSD.ORG>
Cc: Vallo Kallaste <kalts@estpak.ee>, Lars Eggert <larse@ISI.EDU>,
	Poul-Henning Kamp <phk@critter.freebsd.dk>,
	n0go013 <ttz@blahdeblah.demon.co.uk>,
	current <freebsd-current@FreeBSD.ORG>
Subject: Re: ccd performance (was: [ GEOM tests ] vinum drives lost)
References: <62515.1033758160@critter.freebsd.dk> <3D9DEFF7.7050508@isi.edu> <20021005125505.GA1248@tiiu.internal> <20021005221456.GR83766@wantadilla.lemis.com> <20021006072155.GA1117@tiiu.internal> <200210061830.g96IUGNF043142@apollo.backplane.com> <20021007014255.GC93490@wantadilla.lemis.com>
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:>     a lot of information on how to use it.  I generally recommend
:>     using a stripe size of 1152 for multitasking loads.
:
:Sectors?  Why particularly this value?

   It's described in 'tuning'.  Basically you want a fairly large stripe
   to reduce multi-disk seeking when reading sequential files (that is,
   if you do not need the combined bandwidth of more then one drive for
   the sequential case), and you also want to use a stripe that does not
   cause meta-data (e.g. inodes and bitmaps) to wind up on just one drive,
   e.g. use 1152 instead of 1024.  or you will wind up with unbalanced
   accesses.

:ccd, but I know a lot of cheap hardware RAID arrays always read an
:entire stripe at a time, which requires more memory and takes longer.
:Have you checked ccd for this?
:
:Greg

    I've done extensive work on ccd.  It does not try to read a whole
    stripe, it just breaks the I/O up as appropriate, issues duel-I/O
    for mirror writes, and tries to select a reasonable (single) side
    when doing a read from a mirrored area.  I even have a little code
    in there to try to reduce unnecessary seeking when reading from a
    mirrored area.

    But, again, CCD is not trying to implement 'real' RAID.  It can't
    rebuild a lost mirror drive, for example, and does not implement RAID-5.
    IMHO A real RAID controller with NVRAM should be used for those things.

						-Matt


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