Disks and Reservations

Black, David david.black at emc.com
Tue Jun 25 21:45:14 PDT 2013


Formatted message: <a href="http://www.t10.org/cgi-bin/ac.pl?t=r&f=r1306254_f.htm">HTML-formatted message</a>

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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