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[linux-lvm] Re: [PATCH] 64 bit scsi read/write



At 2:39 AM +1200 2001-07-16, Chris Wedgwood wrote:
On Sun, Jul 15, 2001 at 03:44:14PM +0200, Daniel Phillips wrote:

    The only requirement here is that the checksum be correct.  And
    sure, that's not a hard guarantee because, on average, you will
    get a good checksum for bad data once every 4 billion power events
    that mess up the final superblock transfer.  Let me see, if that
    happens once a year, your data should still be good when the
    warrantee on the sun expires.  :-)

the sun will probably last a tad longer than that even contuing to
burn hydrogen, if you allow for helium burning, you will probably get
errors to sneak by

    Surely it can't be that *all* IDE disks can fail in that way?  And
    it seems the jury is still out on SCSI, I'm interested to see
    where that discussion goes.

Alan said *ALL* disks appear to lie, and I'm not going to argue with
him :)

I only have SCSI disks to test with, but they are hot-plug, so I guess
I can write a whole bunch of blocks with different numbers on them,
all over the disk, if I can figure out how to place SCSI barriers and
then pull the drive and see what gives?

Consider the possibility (probability, I think) that SCSI drives blow away their (unwritten) write cache buffers on a SCSI bus reset, and that a SCSI bus reset is a routine, albeit last-resort, error recovery technique. (It's also necessary; by the time a driver gets to a bus reset, all else has failed. It's also, in my experience, not especially rare.)


The fix for that particular problem--disabling write caching--is simple enough, though it presumably has a performance consequence. A second benefit of disabling write caching is that the drive can't reorder writes (though of course the system still might).

At first glance, by the way, the only write barrier I see in the SCSI command set is the synchronize-cache command, which completes only after all the drive's dirty buffers are written out. Of course, without write caching, it's not an issue.
--
/Jonathan Lundell.



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