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December 11th, 2020 09:00
Mixed Drive Sizes
I have inherited a Dell EMC Compellent SC4000. I have been working with HPE 3par for a number of years, so I am trying to port over my knowledge as much as I can. This unit has 36 7K disks, 3 of these are configured as spares and 12 of these are 3.64TB in size and the rest are 1.82TB in size. I also have 10 SSD's that are 1.75TB in size. My current capacity is 95TB however I have used 42TB. It says I have 53TB free; however, when I look at the disks all of my 1.82TB disks are at ~90% capacity and my 3.64TB disks are at ~51% capacity. I have 2 volumes on this array and I am staring to notice performance issues with them. I believe that even though the systems says it has 53TB free I think this is misleading because the extra space on the larger drives is locked because of the smaller drives and everything being in the same (HPE Term) CPG.
What I would like to know is my performance issues because my array is full and the extra space in the larger drives will not be utilized because of the smaller ones. Or is my performance issues because now only the larger drives are storing data as this is the only place and I am going to see major IOPS because of this.
Thanks a bunch for any help with this,
Scott
ScottG67
7 Posts
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December 11th, 2020 14:00
I think I have found an explanation if it is still relevant.
https://www.dell.com/community/Compellent/PERFORMANCE-quot-Short-Stroking-quot-balancing-different-drive/m-p/3844594#M173
Hi everyone
Here's a problem we have run into, and probably affects a number of you too.
The issue is simple - disk drive sizes keep increasing, and it is hard to keep buying the same size of disks for each Tier of storage in your Compellent Storage Center. For example, in our system, we originally purchased 300GB Fibre Channel disks. Compellent stopped selling these in October 2010, so we installed 450GB SAS drives instead.
This means that you have some choices to make about adding new disks which are a different size (larger) to the old ones:
(1) Add the disks as a completely separate disk folder. This means that you are now managing two pools of storage.
(2) Add the disks to the same storage pool.
From an administrative workload as well as space utilisation efficiency point of view, (2) is a better choice.
However, option (2) does present some problems.
If a system has two types of disk, then at some point the smaller disks will fill up, and some data will necessarily be striped across the larger disks and NOT the smaller ones. This effectively results in uneven balancing of the I/O load. The effect is more pronounced in systems which were more full before the disk upgrade took place, since the restripe algorithm gets less efficient the more full the system is already (apparently).
In practices, this means that the system COULD start to experience hot spots where there is an imbalance of drive sizes.
I've discussed this issue with various people at Dell Compellent, and the only obvious solutions to this are option (1) above, or to "short stroke" the new drives.
What do I mean by short stroking in this context?
In other high-end systems, this means confining the disk read head to a particular area of the disk, so that the read head never performs a "full seek stroke" - thus minimising latency. The side effect of this is also to restrict the usable capacity of the disk. In this case, we want to restrict the capacity of the 450GB disks so that they only have 300GB accessible - this will hopefully mean better balancing of I/O across both old and new disks.
So, thanks for your patience in reading this post... please vote below to let me know your views on "short stroking" in the Compellent SAN!
So either my drives have been short stroked and I am out of space or the new data is being written to a small subset of my drives and my IOPS are taking a beating. I do have support on this unit so I might log a call.
Thanks,
Scott
Dell-DylanJ
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2.9K Posts
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December 11th, 2020 14:00
Hello,
It sounds like there may be a bit of a misunderstanding on how storage tiering works in the Compellent ecosystem. I can link you to the Admin Guide, which has a significant amount of information. I think you'll find it a useful tool