Xen users - Feb 2007 - xen: when devices go away...

Hi there,
I''ve been experimenting with iscsi and xen and have found an
interesting
behavior.

I''d reasoned that using iscsi volumes as devices in a software RAID1
configuration would be a Good Idea since when one of the iscsi servers
goes away the filesystem is stable.

I have a dom0 which has access to two iscsi volumes on two seperate
iscsi target servers.

I configure a domU with these two iscsi targets as its block devices, ie:

disk = [
''phy:iscsi/iqn.2006-12.server1:raid/lun0/part,sda1,w'',
        ''phy:iscsi/iqn.2006-12.server2:raid/lun0/part,sdb1,w''
]

root = "/dev/md0 ro"

This works in as much as the domU has root on a RAID1 md device and it
functions normally; until I shutdown one of the iscsi servers. At that
point the domU dies horribly. I see no messages on the domU console at
all and so far nothing on the dom0 console either.

Nothing in any of the logs. I also don''t see anything noteworthy in xm
log nor xm dmesg when this happens; the domU becomes unresponsive and
has to be killed with xm destroy.

Am I expecting too much? Will xen just not cope when one of the block
devices handed out to a domU simply ''goes away''?


iscsi aside, what would xen be expected to do if you had two scsi
drives, sda and sdb, presented to a domU as vbd''s and used by the domU
in a RAID1 array... ie what would xen do if one of those hard drives failed?



Thanks!


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On 2/1/07, Steve Wray <steve.wray@cwa.co.nz>
wrote:
> Hi there,
> I''ve been experimenting with iscsi and xen and have found an
interesting
> behavior.
>
> I''d reasoned that using iscsi volumes as devices in a software
RAID1
> configuration would be a Good Idea since when one of the iscsi servers
> goes away the filesystem is stable.
Note todays thread "Linux software RAID1 (Dom0 or domU)?" ....

jerry

-- 
"Pay no attention to that man behind the curtain!"

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Jerry Amundson wrote:
> On 2/1/07, Steve Wray <steve.wray@cwa.co.nz> wrote:
>> Hi there,
>> I''ve been experimenting with iscsi and xen and have found an
interesting
>> behavior.
>>
>> I''d reasoned that using iscsi volumes as devices in a software
RAID1
>> configuration would be a Good Idea since when one of the iscsi servers
>> goes away the filesystem is stable.
> 
> Note todays thread "Linux software RAID1 (Dom0 or domU)?" ....
Ok I''ve been through that entire thread and it didn''t answer
the
question of what is happening to XEN when the device it presents to the
domU as a vbd vanishes.

This applies just as much to iscsi as to local disk; if I allocate a
whole drive to a domU via XEN say something like:

disk = [ ''phy:hdc,hda,w'',
	''phy:hde,hdc,w'' ]

what happens when hdc in dom0 dies horribly?

Lets leave the issue of whats best practice WRT RAID for now and assume
that the domU has hda and hdc as components of a RAID1 array so that, in
principle, if the dom0 hdc fails everything *should* be ok; can XEN be
expected to be ok with this?



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