FAST cache versus SSD in FAST Pool

My $0.02 cents based on experience in a production enviroment.

I also have a VNX5300 with SSD on both, FastCache and the Pool Itself. But unlike the Vmax that moves the hot data to higher tier on the fly. our VNXs are scheduled. if i were you i would put 2 as fast cache and the 3rd one as hot spare. this would benefit the whole pool, refrain from putting exchange2010 on this vmware pool though: because of the nature of it,you wouldnt want to enable fast cache on the pool where exchange2010 lives (no real benefit and more of a detriment to the rest of the fast cache users due to the way exchange2010 works).

Thanks Renaton for the input!

I have a doubt about IPOS if we use FastCache. Not so sure if you or anyone here calculated also.  I am trying to calculate backend IOPS of my VNX to compare with my old box.  Which I based on the below EMCcalculation. If I put SSDs into my pool, I understand I can add in IOPS based on below roughly. But when it is going to be used by FASTCache. I hear some opinion says it is totally different, one is for IOPS (SSD in the pool), one is for improve response time (FAST Cache), and will not help with IOPS at all?

But basically as I read the difference, what I feel is they are similar thing, promote hot data into SSD, just one is global, move data on fly as mentioned. And another is scheduled.

First time sizing for this, Hope anyone can throw some light here!. thanks

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To determine the number of drive IOPS implied by a host I/O load, adjust as follows for parity or mirroring operations:

• Parity RAID 5:  Drive IOPS = Host read IOPS + 4 x Host write IOPS
• Parity RAID 6:  Drive IOPS = Host read IOPS + 6 x Host write IOPS
• Mirrored RAID 1/0: Drive IOPS = Host read IOPS + 2 x Host write IOPS

I had it calculated as well. Nevertheless its all about what load are you putting onto the VNX array.

let's say SQL server: We cannot meet the IOPS needs for our SQL server but just adding SSDs... it need SPINDLES. So i am finding myself adding 300GB HDDs to meet up my SQL OLTP demands.

I was asked by EMC Design Arquitech about every DB i was going to put on the SQL server, the way it was developed, Read/Writes ratio, 8K or 16K or 4K unit size, imagine i have to call RIM to find out their DB details since my BES servers was using this SQL server. and then i had to create pools and slice LUNS accordingly. All those details were included into the modeling of the VNX array when building the pools. Again depending on the kind of load.

My earlier comment was due to the limited budget you have for adquiring SSDs. So you are correct in assuming the SSDs on the FastCache will not help with the IOPS of the pool (assuming you enable fast cache which can be only done at the pool level). My advice then will be to post what kind of load(s) and IOPS requirments you calculated will be on top of the VNX array and i am sure someone from EMC could quickly run it thru their system and shine some tentative design.

FAST Cache will handle bursty workload and workload that have high locality of reference.. so in fact it will help with IOPS because that workload will be offloaded to SSDs and not pound on your spindles that make up your pool.

Remember that in pools you are dealing with data segmented into 1GB slices, so it will depend on how many hot regions you actually have. If you throw the SSD's in to the pool, yea they will be fast, but that is only going to improve performance of less that 200GB of data assuming 100GB EFD's and you will need additional for an EFD HS.

However, with FAST Cache and DB environments we are often seeing heaving read requirements. In this case, FC will really improve performance reduce the backend IO demand since the request will be served out of cache directly and not from the backend.

This why we see performance improvements in SQL, Oracle and VMware configs by over 110% when Fast Cache is implemented and enabled on hot pools.

i would use them for FAST Cache.  Can you engage your local VNX USPEED guru, they have the tools to help you size your pools/array.

any more details? i read some post say these depends on cache hit, so not that easy to estimate how much it can help IOPS before the cache in use. And since the ssd size small 100GB etc, I am worry that how much it will help for IOPS if i add minimum number of ssd.

Our systems are mainly running SAP systems. I dont really know much details, and it is hard to ask customers to give detail about their DB IOPS, read/write ratio. So what I did is just to calculate old system backend IOPS, to compare with new one

right now we are having 3 old AX4 with total of 51 15k SAS disks, and 20 SATA disks (Raid 5), exclude hotspares and system disks. And these systems are heavily loaded right now

New VNX is having a pool of 38 15k SAS and 14 SATA (Raid6 instead, because SATA are 2TB size ). I think IOPS not enough. Instead of adding one more SAS disk enclosure, I want to add SSD instead, because we dunt need that extra capacity. But again within the budget, 3 ssd are only possible. 

I hope this information are sufficient, and initial sizing for the VNX is not that good. So my question is whether it is more making sense to have these SSD in the pool or in the cache. And any way to calculate roughly these ssd can add IOPS by how much.

VNX properly sized will run circles around your old AX4, without I/O requirements you are guessing.

http://www.emc.com/collateral/hardware/white-papers/h8143-fast-cache-sap-unified-wp.pdf