PERFORMANCE - "Short Stroking" - balancing different drive capacities in the same tier
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.
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
Secondly, if you'd like to send me the serial number in a private message, I can check for you. If the system is in warranty, you also have the option to contact support. They'd be able to look at it with you and answer any questions live.
Let me know if that helps, or if you have any other questions.
ScottG67
7 Posts
0
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
4 Operator
•
2.9K Posts
0
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