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[dm-devel] Re: [RFD] BIO_RW_BARRIER - what it means for devices, filesystems, and dm/md.



Neil Brown wrote:
On Friday June 1, dgc sgi com wrote:
On Thu, May 31, 2007 at 02:31:21PM -0400, Phillip Susi wrote:
David Chinner wrote:
That sounds like a good idea - we can leave the existing
WRITE_BARRIER behaviour unchanged and introduce a new WRITE_ORDERED
behaviour that only guarantees ordering. The filesystem can then
choose which to use where appropriate....
So what if you want a synchronous write, but DON'T care about the order?
submit_bio(WRITE_SYNC, bio);

Already there, already used by XFS, JFS and direct I/O.

Are you sure?

You seem to be saying that WRITE_SYNC causes the write to be safe on
media before the request returns.  That isn't my understanding.
I think (from comments near the definition and a quick grep through
the code) that WRITE_SYNC expedites the delivery of the request
through the elevator, but doesn't do anything special about getting it
onto the media.

My impression is that the sync will return when the i/o has been delivered to the device, and will get special treatment by the elevator code (I looked quickly, more is needed). I'm sore someone will tell me if I misread this. ;-)

--
bill davidsen <davidsen tmr com>
 CTO TMR Associates, Inc
 Doing interesting things with small computers since 1979


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