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

David Chinner wrote:
Sounds good to me, but how do we test to see if the underlying
device supports barriers? Do we just assume that they do and
only change behaviour if -o nobarrier is specified in the mount

The idea is that ALL block devices will support barriers; if the underlying driver doesn't, then the block layer will work around it.

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, so if XFS relies on that block being on the media when the request is completed, then it is broken. It should only care that the ordering of log-data-log is maintained, not exactly when each specific request completes.

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