linux-kernel - Re: [GIT PULL] DRBD bits for 2.6.39

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Date:	Thu, 10 Mar 2011 13:10:38 +0100
From:	Philipp Reisner <philipp.reisner@...bit.com>
To:	Jens Axboe <axboe@...nel.dk>
Cc:	linux-kernel@...r.kernel.org, drbd-dev@...ts.linbit.com
Subject: Re: [GIT PULL] DRBD bits for 2.6.39

[...]
> 
> 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...

Best,
 Phil
-- 
: Dipl-Ing Philipp Reisner
: LINBIT | Your Way to High Availability
: Tel: +43-1-8178292-50, Fax: +43-1-8178292-82
: http://www.linbit.com

DRBD(R) and LINBIT(R) are registered trademarks of LINBIT, Austria.
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