[clug] iSCSI and shared filesystems

Brett Worth brett at worth.id.au
Fri Apr 23 21:35:20 MDT 2010

On 04/24/2010 12:59 PM, steve jenkin wrote:
> Brett Worth wrote on 23/04/10 10:28 PM:
> brett - thanks very much. nice to hear real war stories.
>> I've actually used this.  I was serving LUNs with ietd on Linux and connecting using the
>> iSCSI initiator on Linux too.
>> I've done some experimentation with shared filesystems for Xen migration.  I guess how you
>> do it will depend on what you're trying to achieve with the migration.
> Somehow in previous post and from the on-line "HowTo", I got the idea I
> didn't have to use a shared filesystem within the DomU's. ie. ext3 would
> cut it.

That's right.  What I was getting at is that for deployment of new vm etc having the root
image on a shared filesystem in dom0 makes it simple.

>> Were you thinking of using the iSCSI for the root filesystem?
> Sorry, now confused.
> Root filesystem of Dom0 (host) or DomU (guest)?

I was talking about the domU root filesystem.  So in that case if using iSCSI you can
either have the iSCSI initiator running on the two dom0's and presented as normal scsi
luns.  The alternative I was thinking of would be to have a shared filesystem using some
other method e.g. NFS for the vm images and only use iSCSI for the DB filesystem access.

However you do it it's all good.   Xen/KVM forever!

> My intention was to have local filesystems for DomO and 2 shared
> filesystems for DB-data and DB-logs in the DomU, thinking LVM.

You've actually got to have shared storage for the vm boot images if you want to migrate
be it iSCSI raw luns or on a shared filesystem.

> Not sure how to make & store the DomU images.

I've been using CentOS for all my Xen experiments so I've been using virt-manager which
takes care of image creation etc.

With a year of Xenning behind me there's things I would have done slightly differently if
I was starting from scratch. I'd not use OCFS2 for the shared filesystems on the dom0's
for a start.  I should have persisted with getting GFS to work.  I recently tried using
StorNext from Quantum but the qemu disk subsystem used by KVM and Xen wouldn't run because
of the lack of support for mmap.  i.e.  It couldn't to a loop mount of an image resident
on the cvfs filesystem.


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