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



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

David Lang


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