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PGR limitation

Created: 10 Apr 2012 • Updated: 18 Apr 2012 | 9 comments
This issue has been solved. See solution.

HI all,

Recently, I got a question that does storage array (EMC/IBM/HP, etc) has PGR number limitation? One of my customer said HP EVC supports 512, 128 for HDS...Does PGR has such hardware limitation?

Not sure if my question is valid as I have no idea on this concept of PRG number limitation.

Any input would be very appreciated ! thanks!

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Wally_Heim's picture


The array vender would need to be contacted to check if the specific array in question has a limitation or not.

Thank you,


阿超's picture

So, the array vendor does have a methods to place a limination on PGR if they want, right?

To be honest, I'm not really understand what PGR is, I only know it is a SCSI-3 method to implement I/O fencing. It just looks like a concept, I mean how to make it as a configurable parameter thing likehood on the storage array or cluster side?

Could you explain it to make me more clear? thank you

Eric.Hennessey's picture

Here's the short version: SCSI3 PGR (persistent group reservation) allows hosts to write unique keys to data disks. These are called "write exclusive registrants only" (WERO) keys, and at the array level, only the hosts who hold one of those keys can write to the disks.

There's a SCSI3 PGR command called "preempt-abort" that allows a host to wipe another host's keys from the disks. In such an event, the host whose keys were wiped will no longer be able to write to the disks. In an I/O fencing race, the winning node issues a preempt-abort command on the losing node's key, thus disabling the losing node's ability to write.

Any and all forum readers are invited to fill in any blanks and/or correct me where I'm wrong here.

Business Continuity Solutions Evangelist

阿超's picture

Great point on PGR explanation, but I still don't understand how does the array set limitation onto PGR? Is it doable? Is there any example could make it easy to understand?

g_lee's picture

Is your query similar to what is described in this technote?

ie: when you say "PGR number limitation", you are referring to a limit on the number of PGR keys supported by a particular array?

From the description in the technote, this is a limitation on the storage (array) side, ie: in that case the HDS array had a limit of 128 keys per array port. There may be settings on the array to increase this limit (in the case of the technote, setting host mode option to 61), however this would need to be followed up with the vendor, not Symantec, as it is configured on the array.

Hope that helps.

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阿超's picture

Hi g_lee,

you are right, the limitation is on the array side. In terms of the technode you provide, the limit was set on the array port. Is there any other design which applies the limit on array LUN (device)?? As I know, EMC Symmetrix is set on its device.


g_lee's picture

You still need to check with the vendor if it's set per LUN/device - there may be server settings in conjunction with array settings to get this to work, but the vendor (eg: EMC in your example) needs to advise what is required, as they are the ones presenting the device

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Gaurav Sangamnerkar's picture

I believe it would not be per device, rather would be per port basis only (what I heard of ) however as said by Grace , best would be to check with array vendor ..

thinking of what best can be achieved having control of keys per device or Lun basis ? it may simply result in extra management ..



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阿超's picture

both of you are very helpful, thanks a lot!!