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Re: [Xen-users] GPLPV and LUN's on a storage

To: James Harper <james.harper@xxxxxxxxxxxxxxxx>
Subject: Re: [Xen-users] GPLPV and LUN's on a storage
From: Klaus Steinberger <klaus.steinberger@xxxxxxxxxxxxxxxxxxxxxx>
Date: Tue, 27 Oct 2009 12:17:46 +0100
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Hi James,

I'm not familiar with CX4... is it iSCSI or AoE or hyperSCSI or
Fibrechannel, but supports also iSCSI. For these LUN's I use Fibrechannel.

That is strange... when GPLPV is running as it should, I don't think
that there are any writes done before GPLPV takes over. If there are, it
is done via the int13h BIOS interface and it writes a flag that says the
boot started which enables the next boot to present the "last boot was
not successful... safe mode?" menu.

I think its not the writes, the reads from the VM Caches are the problem, as they are insonsistent with the data on the LUN. That's especially problematic if the Filesystem Metadata is read from cache, but is different to what's on disk. Subsequent writes are probably going into the false places.

Fwiw, I do almost all of my testing on phy: mapped lvm volumes.
Sure, I never seen that problem with LVM. But on a RHEL cluster I can't use LVM Snapshots (they don't work in clustered LVM). So I want to use the LUN handling capabilities of the CX4 (snapshot's, remote mirrorin to a second CX4 and so on).


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