[dm-devel] Re: fastfail operation and retries

[Lars Marowsky-Bree]

On 2005-04-21T17:02:44, "goggin, edward" <egoggin at emc.com> wrote:

> Depending on the "queue_if_no_path" feature has the current undesirable
> side-effect of requiring intervention of the user space multipath components
> to reinstate at least one of the paths to a useable state in the multipath
> target driver.  This dependency currently creates the potential for deadlock
> scenarios since the user space multipath components (nor the kernel for that
> matter) are currently architected to avoid them.

multipath-tools is, to a certain degree, architected to avoid them. And
the kernel is meant to be, too - there's bugs and known FIXME's, but
those are just bugs and we're taking patches gladly ;-)

> I think for now it may be better to try to avoid having to fail a path if it
> is possible that an io error is not path related.

No. Basically every time out error creates a "dunno why" error right now
- could be the storage system itself, could be the network in between.

A failover to another path is the obvious remedy; take for example the
CX series where even if it's not the path, it's the SP, and failing over
to the other SP will cure the problem.

If the storage at least rejects the IO with a specific error code, it
can be worked around by a specific hw handler which doesn't fail the
path but just causes the IO to be queued and retried; that's a pretty
simple hardware handler to write.

But quite frankly, storage subsystems which _reject_ all IO for a given
time are just broken for reliable configurations. What good are they in
multipath configurations if they fail _all_ paths at the same time? How
can they even dare claim redundancy? We can build more or less smelly
kludges around them, but it remains a problem to be fixed at the storage
subsystem level IMNSHO.


Sincerely,
    Lars Marowsky-Brée <lmb at suse.de>

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