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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 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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