Messages in this thread | | | Date | Fri, 01 Jun 2007 19:56:13 -0400 | From | Bill Davidsen <> | Subject | Re: [RFD] BIO_RW_BARRIER - what it means for devices, filesystems, and dm/md. |
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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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