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    SubjectRe: [PATCH] 2.4.x write barriers (updated for ext3)


    On Thursday, February 28, 2002 09:36:52 AM -0600 James Bottomley <James.Bottomley@steeleye.com> wrote:

    > Doug Gilbert prompted me to re-examine my notions about SCSI drive caching,
    > and sure enough the standard says (and all the drives I've looked at so far
    > come with) write back caching enabled by default.

    Really. Has it always been this way?

    >
    > Since this is a threat to the integrity of Journalling FS in power failure
    > situations now, I think it needs to be addressed with some urgency.
    >
    > The "quick fix" would obviously be to get the sd driver to do a mode select at
    > probe time to turn off the WCE and RCD bits (this will place the cache into
    > write through mode), which would match the assumptions all the JFSs currently
    > make. I'll see if I can code up a quick patch to do this.

    Ok.

    >
    > A longer term solution might be to keep the writeback cache but send down a
    > SYNCHRONIZE CACHE command as part of the back end completion of a barrier
    > write, so the fs wouldn't get a completion until the write was done and all
    > the dirty cache blocks flushed to the medium.

    Right, they could just implement ORDERED_FLUSH in the barrier patch.

    >
    > Clearly, there would also have to be a mechanism to flush the cache on
    > unmount, so if this were done by ioctl, would you prefer that the filesystem
    > be in charge of flushing the cache on barrier writes, or would you like the sd
    > device to do it transparently?

    How about triggered by closing the block device. That would also cover
    people like oracle that do stuff to the raw device.

    -chris

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