[Drbd-dev] [GIT PULL] DRBD bits for 2.6.39

Jens Axboe axboe at kernel.dk
Thu Mar 10 14:16:42 CET 2011

On 2011-03-10 13:10, Philipp Reisner wrote:
> [...]
>> Now that I have your attention... Did you look at the plugging changes?
> You always have it :)
> I looked at the changes, and I noticed that we no longer get the unplug
> events. 
>> As Christoph mentioned, you seem to be passing plugging information on
>> the wire. What is the reason for that? With the on-stack plugging, these
>> events are not seen by the block device anymore.
> Imagine DRBD in synchronous mode (protocol C in DRBD speak) on an older
> kernel. We mirror a write, in order to get the write-ack packet from the
> peer, the peer needs to unplug as well. -> Send the unplug events via the
> wire.
> Now, it we would connect a current-head-of-git DRBD on one node to
> a older one (which still needs unplug packets to respond quickly),
> we would have a tar pit block device. (At least for single synchronous
> writes)
> We are in brainstorming mode right now here.
> One idea is to have a timer, that gets touched with every request we get
> in, in case it expires, we send out a unplug event over the wire.
> But having the unplug events would be more elegant of course...

The unplug is essentially the ->request_fn() being run now. So for older
clients you could just always include an unplug even when you pulled
whatever off the queue there is and sent it to the device.

Jens Axboe

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