device mapper not reporting no-barrier-support?

[Anders Henke]

Hi,

I'm currently stuck between Kernel LVM and DRBD, as I'm using Kernel
2.6.24.2 with DRBD 8.2.5 on top of an LVM2 device (LV).

-LVM2/device mapper doesn't support write barriers
-DRBD uses blkdev_issue_flush() to flush its metadata to disk.
 On a no-barrier-device, DRBD should receive EOPNOTSUPP, but
 it really does receive an EIO. Promptly, DRBD gives the
 error message "drbd0: local disk flush failed with status -5".

The physical disk (in LVM speak) is a RAID1 on a 3ware 9650SE-2LP
controller; the driver 3w-9xxx supports barriers and after moving my D
RBD device from the LV to a single partition on the same RAID1, the 
error messages from DRBD vanished.

I've posted a lengty summary of my findings to

http://lists.linbit.com/pipermail/drbd-user/2008-February/008665.html

... where Lars Ellenberg from DRBD basically responded in

http://lists.linbit.com/pipermail/drbd-user/2008-February/008666.html

... that DRBD does catch the EOPNOTSUPP for blkdev_issue_flush and
BIO_RW_BARRIER, but the lvm implementation of blkdev_issue_flush in
2.6.24.2 aparently does return EIO for blkdev_issue_flush.

So simply the question: how should a top-layer driver check wether a lower
device does support barriers? md-raid does check this way differently than
e.g. XFS does, while DRBD also adds a third way to check this.
Or is this "merely" a bug in drivers/md/dm.c?


Anders
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