Disks and Reservations

Ralph Weber roweber at ieee.org
Wed Jun 26 05:21:35 PDT 2013


* From the T10 Reflector (t10 at t10.org), posted by:
* Ralph Weber <roweber at ieee.org>
*
Ah! Some beef to chew on. ...
David Black wrote:
> There are cluster failure cases (e.g., failure of a quorum leader) in 
> which preemption may be used (e.g., by the new leader). Quorum 
> maintenance/management details vary by cluster implementation.
A key point is that the 'new leader' must be sure that everything about 
the old 'quorum leader' is expunged from the disk. A careful look at the 
PREEMPT rules will reveal mountains about what this means.
  * Not only is it critical that the old quorum leader not be allowed to
    access the disk, but ...
  * Everything the disk happens to be doing for the old quorum leader
    must be sent to the ashcan.
David's observation that persistent reservations were built to support 
quorum clusters is spot on.
All the best,
.Ralph
On 6/25/2013 11:45 PM, Black, David wrote:
>
> Kevin,
>
> > I am struggling to understand Reservations in the disk world. Me being a
tape person,
>
> > my mind set is wrapped around protecting the logical position of the
tape. This does
>
> > not seem to be a concern for disks.
>
> That’s correct.
>
> > What are the reasons reservations are used in the disk world? I assume it
is to protect
>
> > a Logical Unit for one applications sole use (at least writing). Are
there other conceptual 
> reasons?
>
> Actually, sole usage is atypical for persistent reservations in the 
> disk world. A significant user of persistent reservations in the disk 
> world is high availability server clustering software where shared 
> reservations are used for cluster quorum maintenance/management on 
> shared storage.
>
> > What are the reasons that a PREEMPT would be used? In the tape world,
PREEMPT is typically
>
> > only used to perform a fail-over from a lost I_T nexus to an alternate
path. Is this the
>
> > same in the disk world, or are there other reasons?
>
> No, there are other reasons. In the disk world, concurrent use of 
> multiple paths is common, as typical use of the SBC command set has no 
> dependence on completion ordering of current commands - if an 
> initiator wants disk command A to complete before disk command B, the 
> initiator generally has to wait for the status from command A before 
> issuing command B.
>
> There are cluster failure cases (e.g., failure of a quorum leader) in 
> which preemption may be used (e.g., by the new leader). Quorum 
> maintenance/management details vary by cluster implementation.
>
> > Also, sometimes in the tape world, Unit Attentions are ignored. Is this
the same
>
> > in the disk world, specifically related to reservations?
>
> Ignoring unit attentions is generally considered poor form in the disk 
> world, but it’s not unheard-of behavior.
>
> Thanks,
> --David
>
> *From:*owner-t10 at t10.org [mailto:owner-t10 at t10.org] *On Behalf Of 
> *Kevin D Butt
> *Sent:* Tuesday, June 25, 2013 4:30 PM
> *To:* T10 Reflector
> *Subject:* Disks and Reservations
>
> I am struggling to understand Reservations in the disk world. Me being 
> a tape person, my mind set is wrapped around protecting the logical 
> position of the tape. This is does not seem to be a concern for disks.
> What are the reasons reservations are used in the disk world? I assume 
> it is to protect a Logical Unit for one applications sole use (at 
> least writing). Are there other conceptual reasons?
> What are the reasons that a PREEMPT would be used? In the tape world, 
> PREEMPT is typically only used to perform a fail-over from a lost I_T 
> nexus to an alternate path. Is this the same in the disk world, or are 
> there other reasons?
> Also, sometimes in the tape world, Unit Attentions are ignored. Is 
> this the same in the disk world, specifically related to reservations?
>
> Thanks,
>
> Kevin D. Butt
> SCSI & Fibre Channel Architect, Tape Firmware
> Data Protection & Retention
> MS 6TYA, 9000 S. Rita Rd., Tucson, AZ 85744
> Tel: 520-799-5280
> Fax: 520-799-2723 (T/L:321)
> Email address: kdbutt at us.ibm.com
> http://www-03.ibm.com/servers/storage/
>
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