Re: Persistent Reservation API V3

From: Christoph Hellwig
Date: Sat Aug 29 2015 - 09:52:35 EST

On Fri, Aug 28, 2015 at 08:33:24PM -0500, Jeremy Linton wrote:
> Hello,
> So, looking at this, I don't see how it supports the algorithm I've been using
> for years. For that algorithm to successfully migrate PRs across multiple paths
> on a single machine without affecting other possible users (who may legitimately
> have PR'ed the same device) I need PR_IN SA 1, READ RESERVATIONS to assure the
> current node owns the reservation before attempting to preempt it on another
> path. This can also assure that the device hasn't been reserved with a legacy
> reservation.

Do you have any code describing this in more detail? We could add the
read side as well if there is strong interest.

> So, this leads me to two more general questions. The first is why isn't the PR
> API simply exported to filesystems as a general reserve/release so that the PR
> happens during mount/dismount. Then DM and friends can be setup to transparently
> migrate or share the reservation, rather than depending on userspace to handle
> these operations...

The API can be used by file systems, and my upcoming NFS SCSI layout
support was the main reason to write this.

> Also, it seems to me the use of CLEAR is extremely dangerous in any environment
> where actual arbitration or sharing of the resource is taking place.

Yes, but having it as a specific API isn't any less dangerous than
having it issued using SG_IO. Reservations really only make sense if
you assume every user of a LU is actually cooperating in some way
and not actively hostile.
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at
Please read the FAQ at