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Re: Persistent Reservation API V2

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  Re: Persistent Reservation API V2 Keith Busch <keith.busch@intel.com> - 2015-08-21 02:00 +0200
    Re: Persistent Reservation API V2 Christoph Hellwig <hch@lst.de> - 2015-08-21 09:10 +0200

#1210798 — Re: Persistent Reservation API V2

On Tue, 11 Aug 2015, Christoph Hellwig wrote:
> This series adds support for a simplified Persistent Reservation API
> to the block layer.  The intent is that both in-kernel and userspace
> consumers can use the API instead of having to hand craft SCSI or NVMe
> command through the various pass through interfaces.  It also adds
> DM support as getting reservations through dm-multipath is a major
> pain with the current scheme.
>
> NVMe support currently isn't included as I don't have a multihost
> NVMe setup to test on, but Keith offered to test it and I'll have
> a patch for it shortly.

Hi Christoph,

I wrote an nvme implementation and it seems to work as expected with your
pr-tests (minor modification to open an nvme target). While API appears to
work as designed, there are a few features of NVMe that are unreachable
with it. For example, NVMe can ignore existing keys when acquiring a
reservation in addition to registering a new key. NVMe can also specify
if whether or not a reservation should persist through power-loss.

Anyway, I don't think SCSI has these same options. Should this new IOCTL
API accommodate the common subset to maintain its simplicity, or make
it more expressive for all features? The more important question might
be if there are any users requiring these features, and I honestly don't
know that right now.
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#1210953

On Thu, Aug 20, 2015 at 11:56:36PM +0000, Keith Busch wrote:

> NVMe can also specify
> if whether or not a reservation should persist through power-loss.

SCSI does as well, it's the APTPL flag.  However reservations not persistent
through a power loss are basically useless, so I decided to force them on
in the API.  From the documentation:

"All implementations are expected to ensure the reservations survive
 a power loss"

So I would prefer not to add it, or if we really have to as a negative flag
to specifically opt out of the APTPL behavior.

> with it. For example, NVMe can ignore existing keys when acquiring a
> reservation in addition to registering a new key.

This sounds like a sensible addition to me, and I wouldn't be surprised
if future SPC versions will add this flag.
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