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

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Message-ID: <e2e108260904140117t274815c6uebe5438ba2bf17b6@mail.gmail.com>
Date:	Tue, 14 Apr 2009 10:17:58 +0200
From:	Bart Van Assche <bart.vanassche@...il.com>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	LKML <linux-kernel@...r.kernel.org>
Subject: Re: dm-multipath and write request ordering

On Tue, Apr 14, 2009 at 9:16 AM, Andi Kleen <andi@...stfloor.org> wrote:
> Bart Van Assche <bart.vanassche@...il.com> writes:
>> - 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 ?
>
> No opinion on that one.

What I'm worried about is that write requests that have been passed to
dm_mp could arrive in another order at the target because of the
variable transport time of protocols like TCP/IP. This could cause
newer data to be overwritten by older data. Separating those write
requests by a barrier won't help because dm_mp ignores barriers.

Bart.
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