linux-kernel - Re: dm-multipath and write request ordering

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Message-ID: <49E750E5.7010106@vlnb.net>
Date:	Thu, 16 Apr 2009 19:38:13 +0400
From:	Vladislav Bolkhovitin <vst@...b.net>
To:	Bart Van Assche <bart.vanassche@...il.com>
CC:	LKML <linux-kernel@...r.kernel.org>, Andi Kleen <ak@...e.de>
Subject: Re: dm-multipath and write request ordering


Bart Van Assche, on 04/13/2009 03:22 PM wrote:
> 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 ?

Not necessary. If replication between B and C done synchronously, 
barriers are not needed. Barriers are necessary only for async commands, 
when the next command sent before the previous one completed.

Vlad
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