disk write barriers

From: Matthias Buelow (mkb_at_incubus.de)
Date: 07/06/05

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    To: freebsd-questions@freebsd.org
Date: Wed, 06 Jul 2005 15:18:53 +0200

    Hi folks,

    I'd like to know something about the (possible) existence of disk
    write-barriers in FreeBSD. I often read the advice that one should
    disable write-back caching on modern disks in order to make softupdates
    actually work. Unfortunately, disabling the write-back cache on typical
    ATA/SATA consumer disks involves a severe performance hit (my Seagate
    SATA drive here drops to 1/5th sequential write speed, probably worse
    for a random workload, and so this workaround isn't really an option.)

    Now I've stumbled across: http://lkml.org/lkml/2005/5/15/88 in which
    someone claims that FreeBSD "used ... write barriers long ago".
    Write-barriers are a kind of synchronization point around critical
    writes (i.e., those which need to be kept in a certain order), where the
    block level drivers disable (or flush) the cache before the barrier, and
    reenable it afterwards. Windows has been doing that for a long time in
    order to make their NTFS halfway reliable (amazing that they got
    something right). Linux apparently also has write-barriers, although for
    SATA only in the most recent 2.6 kernels.

    Now my question is: Does FreeBSD also implements these barriers, or an
    equivalent mechanism, as claimed on the above URL? If it does, why then
    the frequent advice to disable write-back caching? Or is that only for
    the couple drives that ignore any flush cache/disable wb commands? I
    know my drive doesn't ignore that command (since performance drops
    sharply, when I disable the cache), so I would be on the safe side.
    Would it be possible to get an authoritative statement from a FreeBSD
    developer here on that matter?

    Thanks.

    mkb.
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