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Re: [Xen-users] Xen and iSCSI

On Saturday 28 January 2006 1:18 pm, Markus Hochholdinger wrote:
> so the cpu has to work a lot for iscsi. But should performance be better in
> dom0 than in domU? I'm planning to use iscsi in domU, if possible.

if you do heavy network in domU, either it has to pass through the dom0 
kernel, or you have to dedicate a NIC to each domU. so, i think it could be 
slower (or more CPU demanding) to do the initiator in domU than dom0.

> > what i plan to do (once i get a few test boxes) is to use coraid's AoE
> > protocol (the driver is in the main Linux tree).  it's designed to be
> > software driven, without any special card; and it's CPU utilization seems
> > to be around ATA levels.
> I also read about this. My thoughts were that iscsi exists longer and is a
> standard. AoE is no standard and can only be used by linux by now (i
> think). You also can make iscsi targets and initiators without any special
> hardware.

all very true; but software-only iSCSI is a _real_ cpu hog.  i've read about 
40-50% CPU usage for 1Gb on 3GHz P4 (a couple of years ago, might have 
improved with better FSBs or PCI busses)

most 1Gb NICs do some offloading, but only iSCSI ones manage the SCSI part.  
and with the paravirtualized network drivers, i think you can't use either 
TCP or IP processing in the card; at best the card will do only the Ethernet 
frame checksum (i think even 100BaseT NICs do that).  i wouldn't be surprised 
if iSCSI on domU uses twice as much CPU than on dom0 (where it would use the 
full offloading capabilities of the NIC)

> > with any kind of SAN, i think the best strategy would be to use CLVM in
> > dom0 and export the logical volumes to the domUs; this way you only have
> > to setup the SAN drivers in dom0, the domUs would treat those as local
> > disks.
> Is this safe for migration? How can you get the block device, which is
> exported to the domU, to other Xen hosts when migrating an domU? If all

with CLVM you can get several boxes looking at the same SAN drives and seeing 
the same LVs.  of course you can't mount a filesystem on two boxes (unless 
it's a cluster filesystem, like GFS); but its perfectly safe to use a LV for 
a VM on box A, then migrate to box B, where it can find the same LV.  at no 
point in time the LV is used from two different machines (real or virtual) at 
the same time.


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