On Mon, Nov 26, 2012 at 04:49:37PM -0500, Theodore Ts'o wrote:

Christoph, can you give some kind of estimate for the overhead that
adding this locking in XFS actually costs in practice?

I don't know any real life measurements, but in terms of implementation
the over head is:

 a) taking a the rw_semaphore in shared mode for every buffered read
 b) taking the slightly slower exclusive rw_semaphore for buffered writes
    instead of the plain mutex

On the other hand it significantly simplifies the locking for direct
I/O and allows parallel direct I/O writers.

And does XFS
provide any kind of consistency guarantees if the reads/write overlap
spans multiple pages?  I assume the answer to that is no, correct?

The answer is yes as the lock is taken globally on the inode.

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