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

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.

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.

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?

James


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