AW: [DRBD-user] dettach on io-error failed on pending i/o ops

David Engraf engraf.david at
Fri Dec 23 15:16:49 CET 2005

Note: "permalinks" may not be as permanent as we would like,
direct links of old sources may well be a few messages off.

> > When the application cancels i/o requests, the drbd can also cancel
> > the pending requests over the net and should, if there are any more
> > requests on the local device, cancel them too.
> I don't see it is that easy.
> If you see something which I happen to overlook, please provide a patch,
> or "prove of concept" or the like.
> > The problem is as long as there are any pending requests, I can't
> > write to the this ok??
> this is not coded in drbd.
> I did not observe this either, when I did "detach tests".

Yes, when I do a "soft" unplug test with "drbdadm dettach all" it works
fine. This is because the hard disk is no really broken and outstanding i/o
ops were acked after the disk was dettached. But when I do a "hard" unplug,
the i/o ops doesn't get acked (you can see in /proc/drbd under "lo: "), and
the drbd hangs in the function drbd_io_error and waits for completion
(mdev->local_cnt == 0). As long as drbd_free_ll_dev is not called, it seems
that drbd cannot write on the device any more.

> > I think drbd can operate in diskless state, so why don't use this
> > feature?
> we use it.
> some of the relevant code is in drbd_io_error() in drbd_main.c, and as I
> read the code, there should be no more than one second between the two
> messages WARN("Notified peer that my disk is broken.\n");
> and either WARN("Not releasing backing storage device.\n");
> or WARN("Releasing backing storage device.\n");
> unless this is time spent in drbd_md_write(mdev)...
> and I won't throw away this call. it should not block anything, either.
> but maybe we can at least add some "fail fast do not retry" flag here,
> if the lower level drivers support it.

Yes, drbd_md_write waits a long time, this is because it adds a new i/o
write operation which is attached to the queue, due to the timeouts of the
previous i/os these takes some minutes until this requests got the timeout
Now I have reconfigured my drbd to use another disk for the meta file, drbd
doesn't hang and these two messages comes the same second.

> it likely is not drbd who blocks further IO, but the file system (which
> waits for some journal write to complete).
> imho, the solution really is to make the lower level driver
> fail requests faster.

Would be the best thing.

> I may be wrong of course, maybe there is a simple solution on the
> drbd layer, even if it is not our fault. but currently I don't see it...

David Engraf

Virus checked by G DATA AntiVirusKit
Version: AVK 16.2371 from 23.12.2005

More information about the drbd-user mailing list