Concurrent usage of iscsiadm

[Vojtech Juranek]

Hi,
I'd like to follow up with discussion about concurrent usage iscsiadm tool. It
was discussed here about year ago, with suggestion not to use it concurrently
[1]. On the other hand, comment [2] says it should be fine. Is the an agreement
in open-iscsi community if the concurrent usage of iscsiadm is safe or not? If
it's not safe, is there any bug for open-iscsi describing the issue and
potential problems if iscsiadm is used concurrently?

The motivation why I'm popping up this again is that in oVirt project [3] we
use a lock before calling iscsiadm to make sure it's not run in parallel. This
causes us various issues (see e.g. BZ #1787192 [2]) and we'd like to get rid
off this lock.

I run several thousands tests with our typical usage of iscsiadm [4], running
iscsiadm in parallel and haven't spot any issue so far. This suggests removing
the lock can be safe, but of course my tests could be just a pure luck. So
before removing this lock from our code base, I'd like to know your thoughts
about it.

Thanks
Vojta

[1] https://groups.google.com/g/open-iscsi/c/OHOdIm1W274/m/9l5NcPQHBAAJ
[2] https://bugzilla.redhat.com/show_bug.cgi?id=1787192#c18
[3] https://www.ovirt.org/
[4] https://github.com/oVirt/vdsm/blob/master/tests/storage/stress/
initiator.py

[Ulrich Windl]

Hi!

Another thing is: Whether you like systemd or not: It runs many processes automatically and concurrently.
So it seems wise that iscasadm may be run concurrently. If there are issues, iscsiadm should use a MUTEX internally to avoid those IMHO

Regards,
Ulrich
>>> Vojtech Juranek <vjur...@redhat.com> schrieb am 20.10.2021 um 08:58 in
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[The Lee-Man]

Hi Vojtech:

I know there's confusion around this issue, and as some testing at RH has shown, you can get away with using iscsiadm in parallel, as long as you're careful about what you do. For example, if each instance is trying to log into a different target, and there is no error handling occurring, testing seems to show this to be fine.

But I continue to recommend against doing this, because (as I've said before) there isn't sufficient locking in iscsiadm to allow completely parallel execution. [By the way, I'm willing to entertain patches that fix that.]

Yes, some of the code in iscsiadm is safe for parallel execution, such as talking the iscsid and accessing the node database. But much is not, such as error handling and sysfs access.

And there is very little reason to try to login into multiple targets using parallel calls to iscsiadm when iscsiadm now how the "no-wait" option, allowing it to send off login requests without waiting for success or timing out and failing.

Bottom line, I'd say using iscsiadm in parallel at your own risk. And if you do so and find issues, then we can try to address them. But the last thing I want for iscsiadm is one giant lock. And locking individual pieces of iscsiadm can lead to deadlock situations, if not sequenced correctly.

I hope that answers your questions.

[The Lee-Man]

Hi Ulrich:

I don't see how systemd is going to run parallel iscsiadm commands, at least with the units that come with open-iscsi. Systemd actually guards against that.

As far as locking, I'd be glad to entertain/discuss/review patches that make iscsiadm behave well when run in parallel. But such as change isn't on my short list of things to work on, since I don't really see the need. I'd rather focus on making iscsid play well in containers.

On Wednesday, October 20, 2021 at 11:19:24 PM UTC-7 Uli wrote:

[The Lee-Man]

P.S. Perhaps Chris will chime in with his opinion, which may be a bit different than mine on this subject. Chris?

[Ulrich Windl]

>>> The Lee-Man <leeman...@gmail.com> schrieb am 21.10.2021 um 17:55 in
Nachricht <8d711f1e-d3f4-4109...@googlegroups.com>:

> Hi Ulrich:
>
> I don't see how systemd is going to run parallel iscsiadm commands, at
> least with the units that come with open-iscsi. Systemd actually guards
> against that.

Actually my statement was not specific to iscsiadm, but general: If serveral systemd services depend on one systemd target, those services are started in parallel (unless other conditions prevent that) when that target is to be reached.

> https://groups.google.com/d/msgid/open-iscsi/8d711f1e-d3f4-4109-87ed-a950e932
> 9895n%40googlegroups.com.