[clug] iSCSI and shared filesystems

Brett Worth brett at worth.id.au
Fri Apr 23 06:28:28 MDT 2010

On 04/23/2010 09:35 PM, Daniel Pittman wrote:
> Adam Baxter <voltagex at voltagex.org> writes:
>> I'm interested in how you set up a Linux-based iSCSI "host" (I think the
>> terms are different here?). I know my Windows systems can mount drives from
>> an iSCSI server, but looking around leads to software with large price-tags.
> I have not done this myself, but the folks I know who have used iscsitarget
> http://packages.debian.org/search?keywords=iscsitarget

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.

Were you thinking of using the iSCSI for the root filesystem?  If that's the case then I
think you need to have the Xen dom0 machines acting as initiators to what ever is serving
the iSCSI e.g. your NAS box.  So you'd end up with /dev/XX devices on them which the Xen
config files would point to.

I've currently got a pair of Xen machines that run a shared filesystem using OCFS2.  These
are sharing LUNs with Fibre Channel but you could do exactly the same thing with iSCSI.
The OCFS2 is a bit sensitive to storage problems and tends to self-fence (by panicking)
for the slightest reason so you'd need to test it.

If you can use a shared filesystem, even NFS, for the root images it makes maintenance
much simpler..   Maybe get the NAS box to do NFS for the root then share some raw storage
with iSCSI for the database.  You could run into performance problems with the iSCSI.
There's a few recommendations for how to setup the network and tcp offload is at the top
of the list from memory.

Just this week I was also playing with GFS on RedHat but have only used it on a single
node so far.  It's all I could find that could create a 32TB filesystem that actually came
with RedHat.  BTW even though the doco says that ext4 support VERY large filesystems it
seems that the tools (mkfs, fsck etc...) are still limited to 16TB.  Maybe  RHEL6.

Anyway GFS might be a good shared filesystem for Xen machines with iSCSI if you can get
the cman bits to work on a network.


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