Re: [Iscsitarget-devel] Clustered target

[Ross S. W. W. <RW...@me...>]

Kushnir, Michael wrote:
> 
> Good afternoon,
> 
> I’m an avid DRBD user. Recently, there was a lot of talk on 
> the DRBD mailing list about DRBD Dual-Primary with iSCSI and 
> multipath. The problems with such a setup are summarized 
> here: 
> http://fghaas.wordpress.com/2011/11/29/dual-primary-drbd-iscsi-and-multipath-dont-do-that/
> 
> I was wondering If it is possible to add such a feature to 
> IET? Has anyone ever considered such functionality? How much 
> would such an effort cost if someone were willing to sponsor 
> the feature?

In order to have a cluster-aware iSCSI target there would
need to be significant changes in both the configuration side
and operating side.

One change that pops in my head is the coordination of SCSI
reservations between the targets participating. Reservations
could not be granted or revoked until all members approved
the reservation. This means that all IO operations that
depend on frequest RESERVE/RELEASE operations, such as
clustered file systems would hit a triple latency penalty,
or worse for geographically disperse clusters.

One would also need to coordinate configuration of course!

Then there is handling storage failures and you can see
how things start to get way out of control.

The best use of IET and DRBD is in a fail-over scenario.

Have two servers serving up 50% of the storage needs each
off of top-shelf disks, the other 50% of storage off of
slower but still enterprise quality disks, say SAS and
SATA. Have DRBD replicate the SAS on one server to the
SATA on the other server and use something such as
heartbeat to fail-over the targets and IP addresses from
one server to the other in the event of failure.

Each server has multiple interfaces for doing MPIO to
the target, you just don't do MPIO to an active-active
DRBD device across two servers.

For geographically disperse clusters, follow the same
principal as above, but the primary storage is for
in-site hosts and the secondary is fail-over for the
other site. Tricky part here is how to fail-over the
IPs to the other site...

-Ross
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