Re: [Xen-users] Re: Xen + SAN

[Javier Guerra Giraldez <javier@xxxxxxxxxxx>]

On Thu, Jun 23, 2011 at 4:04 PM, Shaun Reitan
<mailinglists@xxxxxxxxxxxxxxxx> wrote:
> I don't know about AOE but with ISCSI the problem I'm seeing is that if i
> create a volume group on the SAN there is no way to export that volume
> group. ÂI could export the device that the volume group was created on but
> then the host doesn't see that VG. I'm assuming i would need to use
> clustered LVM for that which i have been told to stay far away from. Another
> option was to carve out the LV's on the SAN and to export each LV to the
> initiators but still the problem is that the host assigns these luns to
> /dev/sd devices and there's no simple nice way to map them to who owns what
> disk. ÂLeast not that I've found yet. ÂI'm fairly new to ISCSI though so
> maybe there's something I'm missing.


your problem has two parts:

A: the sharing/splitting. iSCSI shares block devices, and (as you've
found) a VG is not a block device.  you have to either export PVs or
LVs

 there are three (main) ways to do it:

1: don't worry about partitions/LVM on the target. Export disks (or
rather, arrays).  On the initator(s) set those as PVs to create a VG
and then LVs.  typically you do it on only one initiator and then
vgscan on all the others (like John Madden).

pros: simple, manageable.  cons: no lock protection, resizing LVs can
be dangerous without suspending initiators.


2: do as 1, but use cLVM.  any LVM management done on any initiator is
automatically and atomically seen by all of them.

pros: safer.  cons: the lock manager itself is a big pain to manage.


3: do all the LVM management on the target(s), export each LV as an
iSCSI LUN.  Most commercial iSCSI appliances include management on par
with LVM, at the very least.

pros: much safer, easier to handle, no need of shared locks.   cons:
you can't have a LV spanning two targets.


B:  the Linux iSCSI initiator exposes each LUN as a full drive:
/dev/sdd, /dev/sde, etc., not as a 'partition' /dev/sdX1, /dev/sdX2,
etc.   That's because they're exactly the same as SCSI disks.  but,
just like SCSI disks, those LUNs are partitionable with any tool.
just do `parted /dev/sdX` and create a full-size /dev/sdX1 partition.

(BTW, wasn't there an iSCSI-specific 'phy:' driver? something like
'phy:iscsi:<lun name>'.... or am i mixing faint memories?)

-- 
Javier

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