[dm-devel] [RFC PATCH 0/4] dm_queue and fastsuspend feature

[Kiyoshi Ueda]

Hello,

I'm working on moving I/O queue of targets to dm core so that
the table can swap/suspend without the queue flushing.
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BACKGROUND

Currently, table swap is needed when we add additional path to
existing multipath map.  To swap table, suspend is needed and
all I/Os are flushed at the suspention time.
If all paths in the multipath map are failed and there are queued
I/Os in the multipath target, these I/Os will return as error
when the I/O flushing.
In such case, user should expect that these I/Os are issued
without error through the new additional path.

To solve the issue, suspend must be done without queue flushing
and the queued I/Os must be handed over to the new table when table
swapping.


DESIGN

The structure of the queue and the queue processing kernel thread
which are used by the multipath target can be generalized so that
other targets can also use it.
So the queue and the thread are placed in dm-core, and targets use it
by using interface functions.
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The following patch set is a draft to solve the issue above.
It is for 2.6.17-rc6-mm1 + Alasdair's patches which was sent
to this ML (Subject: "Next set of device-mapper patches").

The patches are still rough edges and I'd like to get comments
from dm developpers on this approach.
For example,
  1). Queues for a target are placed in "struct dm_target".
  2). Unmap target function is added for fast-suspend feature.
  3). Is the interface sufficient for other target drivers?
      (Current design considers multipath target only.)

Below is TODO list which I'm working on.
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TODO

o If the I/O is splitted across some targets, fast-suspend feature
  doesn't work because the total of queue size doesn't match with
  the number of pending original bios.

o Accounting issue when withdrawing queued bios.

o The dm_queue daemon works for only bio queue.
  Other works should be handled? (e.g. trigger_event of multipath)
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Any other comments are also very welcome!

Regards and thanks for your time,
Kiyoshi Ueda