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[Xen-users] iscsi vs nfs for xen VMs

To: xen-users@xxxxxxxxxxxxxxxxxxx
Subject: [Xen-users] iscsi vs nfs for xen VMs
From: Jia Rao <rickenrao@xxxxxxxxx>
Date: Fri, 7 Aug 2009 09:48:56 -0400
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Hi all,

I used to host the disk images of my xen VMs in a nfs server and am considering move to iscsi for performance purpose.
Here is the problem I encountered:

For iscsi, there are two ways to export the virtual disk to the physical machine hosting the VMs.

1. Export the virtual disk (at the target side , it is either an img file or a lvm) as a physical device, e.g sdc, then boot the VMs using "phy:/dev/sdc".

2. Export the partition containing the virtual disks (in this case, the virtual disks are img files) to each physical machine as a physical device, and then on each physical machine I mount the new device to the file system. In this way, the img files are accessible from each physical machine (similar as nfs), and the VMs are booted using tapdisk "tap:aio/PATH_TO_IMGS_FILES".

I prefer the second approach because I need tapdisk (each virtual disk is a process in host machines) to control the IO priority among VMs.

However, there is a problem when I share the LUN containing all the VM img files among multiple hosts.
It seems that any modifications to the LUN (by writing some data to folder that mounted LUN ) is not immediate observable at other hosts sharing the LUN (In the case of nfs, the changes are immediate synchronized at all the nfs clients). The changes are only observable when I umount the LUN and remount it on other physical hosts.

I searched the Internet, it seems that iscsi is not intended for sharing a single LUN between multiple hosts.
Is it true or ,I need some specific configuration of the target or initiator to make the changes immediately synchronized at multiple initiator?

Thanks in advance,
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