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    SubjectRe: [Patch] document ext3 requirements (was Re: [RFD] Incremental fsck)
    On Tue, 15 Jan 2008 20:24:27 -0500
    "Daniel Phillips" <phillips@google.com> wrote:

    > On Jan 15, 2008 7:15 PM, Alan Cox <alan@lxorguk.ukuu.org.uk> wrote:
    > > > Writeback cache on disk in iteself is not bad, it only gets bad
    > > > if the disk is not engineered to save all its dirty cache on
    > > > power loss, using the disk motor as a generator or alternatively
    > > > a small battery. It would be awfully nice to know which brands
    > > > fail here, if any, because writeback cache is a big performance
    > > > booster.
    > >
    > > AFAIK no drive saves the cache. The worst case cache flush for
    > > drives is several seconds with no retries and a couple of minutes
    > > if something really bad happens.
    > >
    > > This is why the kernel has some knowledge of barriers and uses them
    > > to issue flushes when needed.
    >
    > Indeed, you are right, which is supported by actual measurements:
    >
    > http://sr5tech.com/write_back_cache_experiments.htm
    >
    > Sorry for implying that anybody has engineered a drive that can do
    > such a nice thing with writeback cache.
    >
    > The "disk motor as a generator" tale may not be purely folklore. When
    > an IDE drive is not in writeback mode, something special needs to done
    > to ensure the last write to media is not a scribble.
    >
    > A small UPS can make writeback mode actually reliable, provided the
    > system is smart enough to take the drives out of writeback mode when
    > the line power is off.

    We've had mount -o barrier=1 for ext3 for a while now, it makes
    writeback caching safe. XFS has this on by default, as does reiserfs.

    -chris


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