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set size issue after disk failed
https://3parug.com/viewtopic.php?f=18&t=2657		 Page 1 of 1
Author:  768kb [ Mon Sep 18, 2017 9:30 am ]
Post subject:  set size issue after disk failed
Good day everyone

Does anyone acknowledge or disagree to the following issue, which is still active for OS 3.3.1:

Example:
- 8200 2-node machine
- 8 SSD Drives
- CPG R5 Set size 7+1
- Default Sparing

Now, 8 LDs are created on 8 disks, right. But if one SSD fails now, rumors say 3PAR will have problems creating new LDs because the CPG with 7+1 misses one disk. According to HPE Switzerland guys, this is a know software issue, not solved yet. :shock:

Someone else heard about this? Did / can someone test what happens if this situation comes up?

Thanks & best regards
jan
Author:  JohnMH [ Tue Sep 19, 2017 5:42 am ]
Post subject:  Re: set size issue after disk failed
Prior to express layout the CPG would only allow you to create 3+1 set sizes over 8 disks i.e. a 3+1 set size, with disks split evenly between nodes and LD's created per node so this wasn't an issue.

Now with express layout and set size of 7+1 both nodes access all disks and the CPG properties direct the LD's to grow across 8 disks, if there are only 7 disks remaining the system can't grow and since there's no other tier to grow into writes will eventually stall. The system already has emergency grow rules for other cases but these would need to be modified to fix this particular corner case.

This only effects very small systems using express layout so if you have only 8 disks then 3+1 through 6+1 set sizes would be a better option.
Author:  ailean [ Tue Sep 19, 2017 5:47 am ]
Post subject:  Re: set size issue after disk failed
It has been noted recently, kind of makes sense but I was surprised as "the code is more what you'd call "guidelines" than actual rules" tends to apply to CPG 'rules'. e.g. if the CPG says FC disks but FC disks fill up the CPG will normally start using NL disks for new allocations rather then block a VV growth.

What I really don't like is that the default CPGs the system creates at startup/upgrade on these small configs tend to have these max set size values, so any customers who just go with the defaults are going to run into potential issues. :roll:
Author:  768kb [ Fri Sep 22, 2017 2:15 am ]
Post subject:  Re: set size issue after disk failed
Thank you for your answers, this helps a lot for understanding. We'll evaluate using 6+1 or less for 8-disk systems.
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