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RE: [Xen-users] DomU 2TB vbd limit?

To: Gémes Géza <geza@xxxxxxxxxxxxxxxxxxx>, <xen-users@xxxxxxxxxxxxxxxxxxx>
Subject: RE: [Xen-users] DomU 2TB vbd limit?
From: "Joris Dobbelsteen" <Joris@xxxxxxxxxxxxxxxxxxxxx>
Date: Mon, 26 Nov 2007 23:41:54 +0100
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Thread-topic: [Xen-users] DomU 2TB vbd limit?
>-----Original Message-----
>From: Gémes Géza [mailto:geza@xxxxxxxxxxxxxxxxxxx] 
>Sent: Monday, 26 November 2007 23:17
>To: Joris Dobbelsteen; xen-users@xxxxxxxxxxxxxxxxxxx
>Subject: Re: [Xen-users] DomU 2TB vbd limit?
>Joris Dobbelsteen írta:
>>> -----Original Message-----
>>> From: xen-users-bounces@xxxxxxxxxxxxxxxxxxx
>>> [mailto:xen-users-bounces@xxxxxxxxxxxxxxxxxxx] On Behalf Of Gémes 
>>> Géza
>>> Sent: Monday, 26 November 2007 22:25
>>> To: xen-users@xxxxxxxxxxxxxxxxxxx
>>> Subject: [Xen-users] DomU 2TB vbd limit?
>>> Hi everyone,
>>> Just a quick question:
>>> Is there a limit on the vbd size for pv guest domains, I've found 
>>> only rumors on the Internet about a 2TB limit, but from my 
>>> with
>>> xen-3.0.3 /linux-kernel-2.6.18-5 (installed from a debian etch
>>> packages) I can say that a lvm volume of 3.7TB as created and 
>>> formated in Dom0 is seen as 1.85TB only of course reporting a 
>>> corrupted xfs filesystem ending after the end of the device.
>>> If there is any xen/kernel version where this limitation disappears 
>>> please tell me.
>> This might seem useful:
>> <http://lists.opensuse.org/opensuse-factory/2007-03/msg00327.html>
>> Short version: might be a partition table problem. MS-DOS 
>MFT is limited to 2 TB. Try GPT instead.
>> - Joris
>Unfortunately that is not the problem, there is absolutely no partition
>table et all. The storage is an aoe lblade vith an lvm volume group on
>top of it. On lvm there are a few smaller perfectly working 
>xfs formated
>logical volumes and (currently) one bigger (3.7TB) which is working
>perfectly on Dom0 both as a logical volume and as a filesystem. However
>when I give that logical volume as a physical disk to a DomU 
>that starts
>to balk to me about:
>attempt to access beyond end of device
>hda3: rw=0, want=8283750400, limit=3988783104
>I/O error in filesystem ("hda3") meta-data dev hda3 block
>0x1edbfffff       ("xfs_read_buf") error 5 buf count 512
>XFS: size check 2 failed
>from my computations I've found that the DomU is "seeing" 1.85TB from
>the logical volume it was given.

Unfortunally my knowledge of Xen is way too little to give you any information 
whether there are actually any limitations and if they are different in newer 
versions (but a good thing to add to the "everything" list that should be 
tested beforehand).

A workarround that might work is partitioning the data into smaller parts of, 
say, 1 TB. These can be concatenated using the MD driver by using the "linear" 
personality (not RAIDx). But I agree this would be messy indeed. Probably you 
considered that too already.

You will need others that have more knowledge to answer this for you...


- Joris

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