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Re: [Xen-devel] iSCSI connection corrupts Xen block devices.

On Tue, Mar 26, 2013 at 01:26:24AM -0500, Dr. Greg Wettstein wrote:
> Hi, hope the week has started out well for everyone.
> This report may be in the FWIW department since there may be a
> fundamental reason why this doesn't work.  We elected to report this
> to the Xen community since we thought any behavior which corrupted
> disk images needed to at least be reported.

You are hitting an issue that Roger hit as well. That is the
m2p override mechanism can only handle one override for a PFN - not

Here is the relevant discussion:
> We are maintaining the Xen-SAN release which provides hotplug
> functionality to allow Xen guests to participate as first class
> entities in either an iSCSI or fibre-channel storage network.  We were
> preparing for a second release and ran across behavior which appears
> to cause Xen guest block devices to be corrupted.
> Relevant VM/OS versions are as follows:
>       dom0:   3.4.35
>       domU:   3.4.35
>       Xen:    4.2.1
> The test environment is a domU VM running which is using SCST
> (2.2.0) to export a block device via iSCSI.  An iSCSI connection is
> initiated from dom0 to the target VM.  The iSCSI block device has a VM
> system image on it.  I/O can be done from dom0 to the guest without
> any apparent issues; ie, mounting the filesystem and reading and
> writing to it.
> The problem occurs when a second VM is started which uses the iSCSI
> based block device as its root filesystem.  The VM starts and
> functions normally, I/O can be done without any issues from inside the
> VM.  When the VM is shutdown and the iSCSI connection is closed the
> block device is instantly corrupted.
> The corruption isn't subtle with the begining of the block device
> being over-written with what appears to be generic contents of the
> filesystem.  The corruption doesn't occur when the VM shuts down, only
> when the iSCSI connection is closed.
> If the iSCSI VM target server is run on a separate physical dom0 host
> everything functions normally.  So the corruption is definitely linked
> to the both VM's being run on the same physical dom0 instance.
> The problem occurs regardless of the type of device backend which is
> used for the domU block device exported by SCST.  The behavior has
> been verified with blktap, image over loop and qdisk.  The problem
> also occurs when either FILEIO or BLOCKIO are used for the SCST
> virtual disk.
> As I said at the outset exposing a device to blkback twice may be
> something it was never designed to do.  That being said using VM's for
> this type of testing certainly makes sense and the behavior is
> unexpected.
> Let us know if there are any questions or if additional testing is
> needed.
> Have a good remainder of the week.
> Greg
> As always,
> Dr. G.W. Wettstein, Ph.D.   Enjellic Systems Development, LLC.
> 4206 N. 19th Ave.           Specializing in information infra-structure
> Fargo, ND  58102            development.
> PH: 701-281-1686
> FAX: 701-281-3949           EMAIL: greg@xxxxxxxxxxxx
> ------------------------------------------------------------------------------
> "Laugh now but you won't be laughing when we find you laying on the
>  side of the road dead."
>                                 -- Betty Wettstein
>                                    At the Lake
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