Re: dm mpath: Add timeout mechanism for queue_if_no_path

[Mike Snitzer <snitzer@redhat.com>]

On Thu, Jan 02 2020 at  5:45pm -0500,
Gabriel Krisman Bertazi <krisman@collabora.com> wrote:

> From: Anatol Pomazau <anatol@google.com>
> 
> Add a configurable timeout mechanism to disable queue_if_no_path without
> assistance from multipathd.  In reality, this reimplements the
> no_path_retry mechanism from multipathd in kernel space, which is
> interesting for cases where we cannot rely on a daemon being present all
> the time, in case of failure or to reduce the guest footprint of cloud
> services.
> 
> Despite replicating the policy configuration on kernel space, it is
> quite an important case to prevent IOs from hanging forever, waiting for
> userspace to behave correctly.
> 
> Co-developed-by: Frank Mayhar <fmayhar@google.com>
> Signed-off-by: Frank Mayhar <fmayhar@google.com>
> Co-developed-by: Bharath Ravi <rbharath@google.com>
> Signed-off-by: Bharath Ravi <rbharath@google.com>
> Co-developed-by: Khazhismel Kumykov <khazhy@google.com>
> Signed-off-by: Khazhismel Kumykov <khazhy@google.com>
> Signed-off-by: Anatol Pomazau <anatol@google.com>
> Co-developed-by: Gabriel Krisman Bertazi <krisman@collabora.com>
> Signed-off-by: Gabriel Krisman Bertazi <krisman@collabora.com>

This seems like a slippery slope.

I've heard this line of dm-multipath concern from Google before
(e.g. doesn't want to rely on availability of userspace component).

Thing is, to properly use dm-multipath (e.g. to reinstate failed paths)
multipathd really is needed no?

If not, how is it that the proposed patch is all that is missing when
multipathd isn't running?  I find that hard to appreciate.

So I'm inclined to not accept this type of change.  But especially not
until more comprehensive context is given for how Google is using
dm-multipath without multipathd.

Thanks,
Mike