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



At 3:22 AM +1200 2001-07-16, Chris Wedgwood wrote:
On Sun, Jul 15, 2001 at 08:06:39AM -0700, Jonathan Lundell wrote:

    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.

Is the spec you have distributable? I believe some of the early drafts
were, but the final spec isn't.

I'd really like to check it out myself, I alwasy assumed SCSI had the
smarts for write-barriers and force-unit-access but I guess I was
wrong.

Anyhow, I'd like to see the spec for myself if it is something I can
get hold of.

I was referring to IBM's spec, as implemented in their recent SCSI and FC drives. You can find a copy at http://www.storage.ibm.com/techsup/hddtech/prodspec/ddyf_spi.pdf


WRITE EXTENDED has a bit (FUA) that will let you force that particular write to go to disk immediately, independent of write caching, but there's no suggestion that it otherwise acts as a write barrier for cached writes.

WRITE VERIFY implies a CACHE SYNCHRONIZE, so it's a write barrier, but an expensive (because synchronous) one.
--
/Jonathan Lundell.



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