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

[Jonathan Lundell <jlundell pobox com>]

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.