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Go Back   Linux Archive > Redhat > Device-mapper Development

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Old 08-13-2010, 12:55 PM
Vladislav Bolkhovitin
Default block: replace barrier with sequenced flush

Tejun Heo, on 08/12/2010 04:41 PM wrote:

Each filesystem needs to be updated to enforce request
ordering themselves and then to use REQ_FLUSH/FUA mechanism.

I generally agree with the patchset, but I believe this particular move
is a really bad move.

I'm not mentioning the obvious that a common functionality (enforcing
requests ordering in this case) should be handled by a common library,
but not internally by a zillion file systems Linux has.

The worst in this move is that it would hide all the requests ordering
semantic inside file systems in, most likely, a very much unclear way.
That would lead that if I or someone else decide to implement the
"hardware offload" of requests ordering (ORDERED requests), I or he/she
would not be able to see any improvement until at least one file system
be changed to be able to use it. Worse, if the implementor can't
demonstrate the improvement, how can he encourage file systems
developers to update their file systems? Which, basically, would mean
that only a person with *BOTH* deep storage and file systems internals
knowledge can do the job. How many do you know such people? Both storage
and file systems topics are very wide and tricky, so nearly always
people specialize in one of them, not both.

Thus, this move would basically mean that the proper ordered queuing
would probably never be implemented in Linux.

I believe, much better would be to create a common interface, which file
systems would use to enforce requests order, when they need it.

Advantages of this approach:

1. The ordering requirements of file systems would be clear.

2. They would be handled in one place by a common code.

3. Any storage level expert can try to implement ordered queuing without
a deep dive into file systems design and implementation.

I already suggested such interface in
http://marc.info/?l=linux-scsi&m=128077574815881&w=2. Internally for the
moment it can be implemented using existing REQ_FLUSH/FUA/etc. and
waiting for all the requests in the group to finish. As a nice side
effect, if a device doesn't support FUA, it would be possible to issue
SYNC_CACHE command(s) only for required blocks, not for the whole device
as it is done now.

If requested, I can develop the interface further.


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