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    SubjectRe: [PATCH 5/7] vfs: Add wbcflush sysfs knob to disable storage device writeback cache flushes
    Jens Axboe wrote:
    > On Mon, Mar 30 2009, Jeff Garzik wrote:
    >> Jens Axboe wrote:
    >>> On Mon, Mar 30 2009, Bartlomiej Zolnierkiewicz wrote:
    >>>> On Monday 30 March 2009, Fernando Luis Vázquez Cao wrote:
    >>>>> Add a sysfs knob to disable storage device writeback cache flushes.
    >>>> The horde of casual desktop users (with me included) would probably prefer
    >>>> having two settings -- one for filesystem barriers and one for fsync().
    >>>>
    >>>> IOW I prefer higher performance at the cost of risking losing few last
    >>>> seconds/minutes of work in case of crash / powerfailure but I would still
    >>>> like to have the filesystem in the consistent state after such accident.
    >>> The knob is meant to control whether we really need to send a flush to
    >>> the device or not, so it's an orthogonal issue to what you are talking
    >>> about. For battery backed caches, we never need to flush. This knob is
    >>> useful IFF we have devices with write back caches that STILL do a cache
    >>> flush.
    >> How do installers and/or kernels detect a battery-backed cache that does
    >> not need flush?
    >
    > They obviously can't, otherwise it would not be an issue at all. And
    > whether it's an issue is up for debate, until someone can point at such
    > a device. You could add a white/blacklist.

    Sorry, I guess I misinterpreted your dual "IFF" statement :)

    I completely agree that the suggested knob, for disabling cache flush
    for these battery-backed devices, is at the present time addressing an
    entirely theoretical argument AFAICS.

    Jeff


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