HPE Storage Users Group
https://3parug.com/

Odd number of drives per shelf OK ?
https://3parug.com/viewtopic.php?f=18&t=2670		 Page 1 of 2
Author:  markinnz [ Sun Oct 01, 2017 9:59 pm ]
Post subject:  Odd number of drives per shelf OK ?
Hi all,

We're getting a number of 2 node 8440s delivered with what looks like an odd configuration to me.

total of 4 drive shelves (shelf with nodes + 3 extra drive shelves) and 20 * 3.84TiB SSDs.

Expected configuration is mean to be RAID-5 3+1 HA

Is that a valid config ? I thought drives had to be put in pairs per shelf ? (from the best practises guide) I did ask the HPE person and got told to read up on "Express Layout", but even though that allows the 2 nodes to share PDs when creating LDs it still doesn't feel like it would allow an odd numbner of drives per shelf.

Anyone have comments ? Or a good link to "Express Layout" as I can only find the same paragraph repeated in multiple diocuments.

I may be over thinking things! :-)

Code:
Express Layout
This unique technology born from the HPE 3PAR three-layer virtualization technology allows 3PAR controller nodes to share access to SSDs in
order to further drive efficiency. Replacing traditional layouts for flash, Express Layout allows each SSD to be actively accessed by both
controllers in a node pair at the same time. This allows a node pair to use capacity from every drive to build logical capacity. For smaller
configurations, like an eight drive system, Express Layout allows the nodes to significantly reduce the overhead historically associated with parity
RAID layouts and can result in more than 10% reduction in overhead which increasing performance by allowing more than one controller to drive
I/O through the drive
Author:  MammaGutt [ Mon Oct 02, 2017 12:39 am ]
Post subject:  Re: Odd number of drives per shelf OK ?
Last time I checked you shall install disks in pair in each shelf, ie you need an even number of drives. Looks almost like someone either forgot that rule, forgot a cage or added 4 more drives in the last minute.

3+1 with HA cage will only make use of the capacity of. 16 drives in your scenario. Next step would be 24.
Author:  markinnz [ Thu Oct 05, 2017 3:59 am ]
Post subject:  Re: Odd number of drives per shelf OK ?
Well I deleted an earlier reply of mine to this thread ... the array got installed with 24 drives ("yay" I thought!) however 4 drives were not ours and the engineer installed them by mistake.

So we have 20 drives now, 2 shelves with 6 and 2 with 4 ... RAID5 3+1 HA works in that config, but it's not exactly elegant, and I feel that the drive shelves with 4 drives in will run higher workload and used capacity than the other 2.
Author:  JohnMH [ Thu Oct 05, 2017 7:41 am ]
Post subject:  Re: Odd number of drives per shelf OK ?
With HA cage you'll lose some of the potential capacity (chunklets) as the disks in the enclosures containing 6 drives will have to write part of their remaining stripe and parity to the enclosures with 4 drives and this will in turn rotate around in order to keep the LD's on separate cages. Ideal for 3+1 would have been 24 drives or maybe stick with 20 with 4+1 & HA Mag or 4+1 and add another cage. The loss should be minimal and only when reaching capacity limits but if you want to stick with HA cage I'd consider adding another 4.

With 3+1 express layout doesn't even come into play, it's really if you wanted to only buy say 8 disks on a 2 node and run say 7+1.
Author:  Noss [ Mon Oct 09, 2017 6:53 am ]
Post subject:  Re: Odd number of drives per shelf OK ?
Slightly different question but thought I would tag on. I've inherited looking after system that is currently laid out as in image attached.

Customer has asked about adding some more 450GBs (These are filtered for use by one CPG in a 3+1 HA and currently 80% used).

What minimum amount of 450GB disks would you install and where?

Attachments:
DriveLayout.JPG
DriveLayout.JPG [ 91.68 KiB | Viewed 21314 times ]
Author:  JohnMH [ Mon Oct 09, 2017 10:13 am ]
Post subject:  Re: Odd number of drives per shelf OK ?
I'm assuming this is a 2 Node with HA cage ?

You have 108 x 450GB / 2 nodes = 54 / set size of 4 = 13.5 so not quite balanced.

If you add another 4 drives = 112 / 2 nodes = 56 / 4 = 14 sets per node

Then you could add another 8 (top or bottom) just ensuring they are on different enclosures to those in slots 0,1,2 & 3

120 / 2 nodes = 60 / set size of 4 = 15 sets per node.

Attachments:
cage.jpg
cage.jpg [ 238.17 KiB | Viewed 21299 times ]
Author:  Noss [ Mon Oct 09, 2017 10:26 am ]
Post subject:  Re: Odd number of drives per shelf OK ?
Sorry. It's a 7200. so 2node.
Author:  Noss [ Mon Oct 09, 2017 11:05 am ]
Post subject:  Re: Odd number of drives per shelf OK ?
Thanks JohnMH. Yes, you assumed right.
Author:  MammaGutt [ Mon Oct 09, 2017 12:20 pm ]
Post subject:  Re: Odd number of drives per shelf OK ?
I would look at this differently and ask. How much space do you need and are you planning on growing more after that? If you plan to add more 900GB drives (if you still get those) I would probably stay away from the first 4 cages. I would somewhat agree on the numbers 2 + multiple of 8 (no express layout in play here so need full stripes per node). If you are going for 18 I would consider a new cage to prevent your next upgrade from being a pain trying to stay somewhat balanced. Anyways, this is a 2-node system so unbalanced hardware shouldn't bite you that hard in terms of chunklet distribution.
Author:  JohnMH [ Mon Oct 09, 2017 1:37 pm ]
Post subject:  Re: Odd number of drives per shelf OK ?
Agreed, plenty of options available.
Page 1 of 2 All times are UTC - 5 hours
Powered by phpBB © 2000, 2002, 2005, 2007 phpBB Group
http://www.phpbb.com/