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RE: [Xen-users] AoE (Was: iscsi vs nfs for xen VMs)

To: "Simon Hobson" <linux@xxxxxxxxxxxxxxxx>, <xen-users@xxxxxxxxxxxxxxxxxxx>
Subject: RE: [Xen-users] AoE (Was: iscsi vs nfs for xen VMs)
From: "James Harper" <james.harper@xxxxxxxxxxxxxxxx>
Date: Fri, 28 Jan 2011 20:57:59 +1100
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Thread-topic: [Xen-users] AoE (Was: iscsi vs nfs for xen VMs)
> James Harper wrote:
> > I use DRBD locally and used to regularly see messages about
> > outstanding requests to the same sector. DRBD logs this because it
> > guarantee the serialising of requests so two write requests to the
> > sector might be reordered at any layer different between the two
> > servers. It sounds like AoE would make this even worse if the
> > write was lost resulting in the 'second' write being performed first
> > followed by the 'first' write.
> Bear in mind that with modern disks it is normal for them to have
> command queuing and reordering built in. So unless you specifically
> turn it off, your carefully ordered writes may be re-ordered by teh
> drive itself.

That's why barriers were invented. I don't see how AoE could possibly
support that if it just processes requests in the order it receives
them, eg if the command sequence went:

Write2 w/barrier

If Write2 was lost by the network and AoE just writes the writes as it
receives them then we have a problem. It must implement some sort of
sequencing and ordering or data loss is inevitable unless you can
guarantee that nothing ever crashes or fails (in which case why are you
using RAID, journaling filesystems, etc).


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