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is a 14+2 config on an 8440 with 32 drives valid?
https://3parug.net/viewtopic.php?f=18&t=2831		 Page 1 of 2
Author:  Kyle [ Fri Mar 23, 2018 10:18 am ]
Post subject:  is a 14+2 config on an 8440 with 32 drives valid?
In another post on here I saw a warning about making a raid config that's the same as the number of drives is dangerous because of problems that happens when you lose a drive.

I have a new 8440 and am setting it up and was told I can do 14+2 on this. I pushed back and said I saw a wrning on a forum about having raid sizes that match the # of drives and they said on an 8200 that would be valid concern for this number of drives but not on an 8440. Is this correct?

In short:

8200 with 32 drives: 6+2 is ok, 14+2 shouldn't be used
8440 with 32 drives: 6+2 is ok, 14+2 is ok (same performance as 6+2 but better space usage)

Is this accurate?

Thanks
Author:  JohnMH [ Fri Mar 23, 2018 6:21 pm ]
Post subject:  Re: is a 14+2 config on an 8440 with 32 drives valid?
Depends on a few factors but it's more applicable to very small systems e.g. With 2 nodes and 14+2 with only 16 disks and express layout, if a disk were to fail then the system can no longer build 14+2 stripes on the remaining 15 disks.

With more disks than the stripe size then it's not an issue as the CPG can continue to grow across the remaining disks, or node pairs if you have more than two nodes. So ideally on these smaller configurations you'd want a smaller stripe size to ensure this can't happen.

Prior to express layout this couldn't happen as disks were dedicated per node, so for 14+2 and two nodes you used to need a minimum of 32 disks. Express layout enables both nodes to build logical disks across the same physical disks attached to that node pair so fewer disks are now required and hence this recommendation for minimal drive configurations.

Hope that makes sense.
Author:  Richard Siemers [ Sat Mar 24, 2018 5:56 am ]
Post subject:  Re: is a 14+2 config on an 8440 with 32 drives valid?
I'm confused.... something is backwards (could be me!).

What's wrong with an 8200 and 32 drives using 14+2? That's an even 16 drives per node, no express layout required.

With a 4 node 8400 and 32 drives, 14+2 isn't possible unless you have 64 drives. 6+2 evenly splits the PDs across all 4 nodes. 32/4=8 PDs.
Author:  JohnMH [ Sat Mar 24, 2018 6:21 am ]
Post subject:  Re: is a 14+2 config on an 8440 with 32 drives valid?
Either is possible.

There's no issue with 2 nodes 32 disks and 14+2.

Not sure if node count was mentioned on the original post.

With 4 nodes and 32 disks running 14+2 the system would use express layout to build the logical disks with each node pair placing chunklets across the same disks. If you have a disk failure on one node pair in theory the other node pair will continue to allow LD growth until the failed disk is replaced. But because you have such a large stripe size and limited disk per node pair that means one node pair can no longer service new writes until the disk is replaced meaning it's not a good idea.

So for 2 node and 32 disks you're good, but on a 4 node l would sacrifice some capacity and go 6+2 or if you're using mag rather than cage you could do 8+2 or 10+2 and lose a few chunklets of capacity.

Rebuilds and upgrades can also be more costly on larger stripe sizes so even though it's possible it's about acheiving the right balance and there are quite a few variables in play, e.g. mag, cage, node count etc.
Author:  Richard Siemers [ Sat Mar 24, 2018 6:45 am ]
Post subject:  Re: is a 14+2 config on an 8440 with 32 drives valid?
JohnMH wrote:
But because you have such a large stripe size that means one node pair can no longer service new writes until the disk is replaced.


Lets drill down on that. New writes or new allocations? A LD has space already allocated to it, most of it will be allocated to a VV, but some of it will be free. Since raid6 is done at the LD level, I can see how the system would not be able to create a new LD... possibly not be able to grow an LD, but I would hope that writes into space already allocated to the LD would still work.

Either way, it looks like using Express Layout will need some extra care to watch for LD's out of balance across nodes after a disk is replacement.
Author:  JohnMH [ Sat Mar 24, 2018 6:49 am ]
Post subject:  Re: is a 14+2 config on an 8440 with 32 drives valid?
Yes there would be some limited space avail for new writes but really it's a moot point as there's no way to grow the LD until the physical disks again match or exceed the stripe size.

However it's worth noting this is only an issue if you are running the absolute minimum number of disks required to service the requested layout.
Author:  Richard Siemers [ Sat Mar 24, 2018 6:51 am ]
Post subject:  Re: is a 14+2 config on an 8440 with 32 drives valid?
Just an afterthought, Express Layout was added to help the 3PAR compete and fit into smaller deployments. I believe that a 8200 with Express Layout overlaps/overextends into Nimble's sweet spot.
Author:  JohnMH [ Sat Mar 24, 2018 6:57 am ]
Post subject:  Re: is a 14+2 config on an 8440 with 32 drives valid?
Agreed if your running all flash 8200 with full on data services you would have been better investing in the higher end controllers to take full advantage of the media, which probably makes up the vast majority of the cost for all flash systems.
Author:  Richard Siemers [ Sat Mar 24, 2018 7:24 am ]
Post subject:  Re: is a 14+2 config on an 8440 with 32 drives valid?
JohnMH wrote:
Yes there would be some limited space avail for new writes but really it's a moot point as there's no way to grow the LD until the physical disks again match or exceed the stripe size.


I am trying to discriminate between new writes, and overwrites of existing data, tempdb for example, to gauge the impact of operating with a failed disk.

JohnMH wrote:
There's no issue with 2 nodes 32 disks and 14+2.


Does that imply using express layout or traditional layout? If that setup works with express layout, then each node would have enough PDs for 2x stripes and avoid the new allocation issue... but if express layout is not used, then it would be a problem right?
Author:  JohnMH [ Sat Mar 24, 2018 10:39 am ]
Post subject:  Re: is a 14+2 config on an 8440 with 32 drives valid?
At minimal disk numbers in theory all writes but there is logging space to accommodate limited writes. Once filled you either have to adjust the stripe size, fail the write or begin parity shedding as you no longer have space for parity on a different drive which means new or updated data would have no parity protection.

In regards to the 2 node 32 disk once you get over the minimum disk numbers required to satisfy the stripe size it doesn't matter whether traditional or express as they become pretty much the same. The only difference being with express LDs can be built by all nodes across all disks, which means ongoing upgrades can also be smaller.

E.g
Traditional 7+1 with 2 nodes would need 16 disks.
Express would need 8 disks minimum, so a failure with only 8 disks is an issue because you can't write 7 data 1 parity redundantly to only 7 disks.
If you have more than 8 disks then it's a none issue and if you have 16 disks then the two are in effect identical.
Taking it a step further if you had 16 disks @7+1 then the traditional layout would require another 16 as the next upgrade (8 per node) for a total of 32. With express since both nodes can use the same disk for LDs this would require only 8 additiinal disks for a total of 24.

So in the above with express and only 8 disks in the array, 6+1 or less would be a better option, but this only really applies with absolute minimal disks as there's no physical spare spindle to replace the failed disk, only spare space on the remaining disks and you don't want data and parity from the same stripe on the same disk.
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