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Mixed drive sizes in a CPG?
https://3parug.net/viewtopic.php?f=18&t=3168		 Page 1 of 2
Author:  richard612 [ Tue Apr 23, 2019 5:45 pm ]
Post subject:  Mixed drive sizes in a CPG?
The 3PAR manuals advise against this as the smaller drives will fill up first causing new data to be localized to the larger drives, reducing the spindles involved in accessing said data, and resulting in a performance hit. Is this still an issue in OS 3.2.2? Should I strive to keep all CPG's strictly limited to a specific drive size?
Author:  MammaGutt [ Wed Apr 24, 2019 3:10 am ]
Post subject:  Re: Mixed drive sizes in a CPG?
This is a physical limitation that no software code can solve. A drive has a given IOPS depending on certain factors (primarily rotation speed of the platters). A 600GB 10k SAS drive can handle the same amount of IOPS as a 1.8TB 10k SAS drive. The big difference is that (given 180 IOPS per 10k drive, I think HPE is saying 150) you have 0.1 IOPS per GB (or 100 IOPS per TB) on a 1.8TB drive, while you have 0.3 per GB (or 300 IOPS per TB) on the same 600GB 10k SAS drive. So the 600GB drive can handle 3 times as hot data as then 1.8TB drive.

Further it can also cause trapped space if you as an example have 100x 600GB drives and add 6x 1.8TB drives with R5 7+1 but as long as you add at least equal to the set size per node pair (for 3.3.1, 2x set size for 3.2.2 and older) that shouldn't be any capacity issue. From a performance it might be as when then 600GB drives are full, the 1.8TB drives need to handle all new writes.

Generally I think of this as an issue for spinning media but you could get into some issues with SSDs as well if there are really big differences.
Author:  richard612 [ Wed Apr 24, 2019 8:07 am ]
Post subject:  Re: Mixed drive sizes in a CPG?
Thanks! This is an excellent answer.

We have a 7400 with 16x SSD; 8 are smaller SLC and 8 are larger MLC. I'm trying to figure out the best way to leverage these drives. Do you know if adaptive flash cache can be configured to utilize only the SLC drives? I was thinking to leverage the MLC drives as tier 0 in an AO.
Author:  MammaGutt [ Wed Apr 24, 2019 12:03 pm ]
Post subject:  Re: Mixed drive sizes in a CPG?
I wouldn't be that worried about mixing SSDs.

I doubt you would be able to hit any IOPS per GB issues with those.

Is it a 2 node or 4 node? And 7400 or 7400c?
Author:  richard612 [ Fri Apr 26, 2019 8:47 am ]
Post subject:  Re: Mixed drive sizes in a CPG?
7400 4-node.

So on our other 7400, I did notice that a CPG encompassing three different disk sizes seems to be filling them with equal amounts of data. The 450s (128 spindles) are 84% full, the 900s (166 spindles) are 44% full, and the 1.2's (114 spindles) are 33% full. So in the not-too-distant future the 450s will be full and all the all new writes will have to go to the remaining spindles. That's 128 spindles worth of IOPs we're going to lose for the new data.

I can see that we need to get this divided up into three separate CPGs. This unit isn't licensed for tunevv so I've got quite a bit of work ahead of me. :cry:
Author:  MammaGutt [ Fri Apr 26, 2019 10:57 am ]
Post subject:  Re: Mixed drive sizes in a CPG?
richard612 wrote:
7400 4-node.

So on our other 7400, I did notice that a CPG encompassing three different disk sizes seems to be filling them with equal amounts of data. The 450s (128 spindles) are 84% full, the 900s (166 spindles) are 44% full, and the 1.2's (114 spindles) are 33% full. So in the not-too-distant future the 450s will be full and all the all new writes will have to go to the remaining spindles. That's 128 spindles worth of IOPs we're going to lose for the new data.

I can see that we need to get this divided up into three separate CPGs. This unit isn't licensed for tunevv so I've got quite a bit of work ahead of me. :cry:


Now, I am lazy... Assuming 900/1.2 can handle the IOPs, I would modify existing CPG to only contain drives with more than 700 chunklets, create a new CPG for 450GB drives and run tunesys. That will "throw" the 450GB drives out of that CPG and you've completed the split.
Author:  Patrick [ Sun Apr 28, 2019 6:50 am ]
Post subject:  Re: Mixed drive sizes in a CPG?
We originally ordered 900GB SFF drives, but as they fail HPE has been sending us 1.2TB and never mentioned any issues.

Given we generally run out of IOPS at the 10k tier before space, i'm not really worried either.
Author:  ailean [ Mon Apr 29, 2019 4:49 am ]
Post subject:  Re: Mixed drive sizes in a CPG?
We had this when they phased out the 450GB 15k drives, all replacements were 600GB and on our T800 that meant 4x600GB in a Mag for every 450GB drive failure (yay free upgrade :) ).
For us 450/600 on a 15k drive matched well with our IOPs/GB load and as the difference wasn't huge it didn't cause many issues (apart from when the 450s all filled up :roll: ).
The 3PAR seemed to slightly weigh the distribution towards the bigger drives in the CPG so we were typically hitting ~90% total array used before the 450s filled, which is not a good place to be in any thin array and was mostly down to delays in our internal order processes.
If the replacements were double or more the original drives it would be more of an issue (I'd look at CPG filters then and maybe tiering the smaller drives as a 'better' tier then the bigger ones, IOs per GB wise).
Author:  MammaGutt [ Mon Apr 29, 2019 6:01 am ]
Post subject:  Re: Mixed drive sizes in a CPG?
Or looking at it from another (simplified) point of view.

You have 100x 900GB drives (90TB total) with 200 IOPS per TB and a total backend capacity of 18000 IOPS. 25 drives failed and are replaced with 1.2TB drives you still can do 18000 backend IOPS but you have more capacity. The array will still be able to do 200 IOPS per TB for a total of 90TB but you might get into trouble if/when you go past 90TB (you originally had and bought) and utilize more than 900GB of the 1.2TB drives and they handle all the writes.

If the load requires 200IOPS per TB then one needs to ensure that one upgrade the array before all drives hit 900GB, if the load is less you get a free upgrade.
Author:  richard612 [ Mon Apr 29, 2019 11:16 am ]
Post subject:  Re: Mixed drive sizes in a CPG?
This is good information. Thanks everyone!
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