Messages in this thread | | | Date | Tue, 29 May 2007 17:01:24 -0700 (PDT) | From | david@lang ... | Subject | Re: [RFD] BIO_RW_BARRIER - what it means for devices, filesystems, and dm/md. |
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On Wed, 30 May 2007, David Chinner wrote:
> On Tue, May 29, 2007 at 04:03:43PM -0400, Phillip Susi wrote: >> David Chinner wrote: >>> The use of barriers in XFS assumes the commit write to be on stable >>> storage before it returns. One of the ordering guarantees that we >>> need is that the transaction (commit write) is on disk before the >>> metadata block containing the change in the transaction is written >>> to disk and the current barrier behaviour gives us that. >> >> Barrier != synchronous write, > > Of course. FYI, XFS only issues barriers on *async* writes. > > But barrier semantics - as far as they've been described by everyone > but you indicate that the barrier write is guaranteed to be on stable > storage when it returns.
this doesn't match what I have seen
wtih barriers it's perfectly legal to have the following sequence of events
1. app writes block 10 to OS 2. app writes block 4 to OS 3. app writes barrier to OS 4. app writes block 5 to OS 5. app writes block 20 to OS 6. OS writes block 4 to disk drive 7. OS writes block 10 to disk drive 8. OS writes barrier to disk drive 9. OS writes block 5 to disk drive 10. OS writes block 20 to disk drive 11. disk drive writes block 10 to platter 12. disk drive writes block 4 to platter 13. disk drive writes block 20 to platter 14. disk drive writes block 5 to platter
there is nothing that says that when the app finishes step #3 that the OS has even sent the data to the drive, let alone that the drive has flushed it to a platter
if the disk drive doesn't support barriers then step #8 becomes 'issue flush' and steps 11 and 12 take place before step #9, 13, 14
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