FAST Cache VNX2 series

I would suggest you take a look at the "EMC VNX2 Unified Best Practices for Performance" white paper for some better direction on the subject.

With 8 usable FAST Cache disks, going wide across the back-end SAS ports and/or the number of Enclosure 0's you have as part of your overall configuration may result in a better balanced approach. Granted, there are many "it depends" which probably have more to do with the particular VNX2 model being deployed and the expected workload profile of the environment... No easy answers here...

However, if you can tell the community more about your actual VNX2 environment and anticipated workload(s), you may get more suggestions to take into consideration...

@Alexander - Thank you. That is more for Performance, whereas my question is more of placement.

Yes, I did go through the whitepapers - all related to FAST Cache for VNX2 series. My question is not related to the  # of disks per bus (which is recommended to be 8 per bus), but more of the placement.

We have multiple VNX1 and VNX2 arrays. The one I'm asking about is for a Block Only VNX8000 currently with 9 FAST cache disks (8 usable) with 35 more to expand. (total usable = 40, hot spares = 3, unusable = 1).

In the VNX1 series, FAST cache disks were only recommended to be placed on Bus 0 Enclosure 0, hence my query of if that is still the case with VNX2 series arrays.

Basically we will be going from

8 disks = ~730GB of FAST Cache

to

40 disks = ~3550GB of FAST Cache.

Understood and thank you for the additional details about your specific array environment.

I suspect we would both agree that "placement" can ultimately affect "performance"...

I would also suggest that the best practice recommendations for FAST Cache in a VNX2 array are different from what they were in a VNX1 array given that both the H/W and S/W (MCx) architecture has been changed...

In a block VNX8000 array you can have up to a maximum of 16 x back-end SAS ports per storage processor (SP) depending on the actual array configuration...

Going wide with the FAST Cache disks across the total number of "Bus X Enclosure 0" enclosures that make up the array configuration would be the better performance recommendation.

Placing a greater number of FAST Cache disks on a fewer number of back-end SAS ports can potentially lead to contention.

Since the FAST Cache is a global resource within the array, you do not want it to become the critical path item in regard to overall array performance...

I hope this answers your question...

Thank you Alexander. I agree with palcement and performance - but in my case, if I cannot use (place) the disks, there is no performance impact since they are 'unbound'

I agree with you on going wide and then going deep. I thought the same but did not want to Destroy existing and Recreate new FAST Cache without checking on any aspects that I need to check on.

I will be performing this within the next couple of months, but this is essential since it gives me planning time for cabinet placements (we are out of space in our current VNX8000 cabinet and need to expand, which is where 16 of these disks will go). Remaining 16, I guess will need to re-configure at a later date.

Thanks!

You're welcome!

I agree that "if I cannot use (place) the disks, there is no performance impact since they are 'unbound'"... However, keep in mind that even though the EFDs may not be used as FAST Cache drives, they may still be used for other purposes in the array so placement will still matter one way or another...

And, going through the process of "Destroy existing and Recreate new FAST Cache" will typically require a fair amount of time and may impact on-going workload since the FAST Cache is fully drained and after recreation has to be re-warmed...

Either way best wishes down the road...