dm-multipath and write request ordering

[Bart Van Assche]

Hello,

Several people are using the dm-multipath software as follows:
* Linux server A is using dm-multipath to access data stored on
servers B and C via two iSCSI sessions -- one session between servers
A and B and one session between servers A and C.
* On servers B and C iSCSI target software exports a block device that
is replicated between servers B and C.

Round-robin load balancing will only work correctly in such a setup if
the replication software knows the order in which write requests have
been queued on the dm-multipath device. Since iSCSI uses the TCP/IP
protocol, write requests generated by server A can arrive out-of-order
on servers B and C. My questions are as follows:
- Is it correct that round-robin load balancing can only work
correctly in such a setup with proper support for write barriers in
the device mapper ?
- Did I understand it correctly that the current dm implementation
only supports barriers when remapping a single underlying device ?
- Are there any plans to add barrier support to dm-multipath ?

This is where I obtained my information about device mapper from:
[1] Jonathan Corbet, Multipath support in the device mapper, February
2005, http://lwn.net/Articles/124703/.
[2] Andi Kleen, [PATCH] Implement barrier support for single device DM
devices, February 2008, http://lkml.org/lkml/2008/2/15/125
[3] Stefan Bader, [2.6.23 PATCH 11/18] dm: disable barriers, July
2007, http://lkml.org/lkml/2007/7/11/432.
[4] dm source code,
http://git.kernel.org/?p=linux/kernel/git/stable/linux-2.6.29.y.git;a=tree;f=drivers/md;h=e0a3c2abb32bcec01c192217cda3f085e91217f3;hb=HEAD.

Bart.
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