Is SSD read cache going to help?

Have a Hyper-V environment - 4 hosts sharing a single Powervault md3800f with 10 x 15k SAS in RAID10 connected via redundant fibre channel.

Average IOPS, latency, throughput, queue depths etc. all very much acceptable and actually lower than expected.  But I'm seeing regular spikes and the odd complaint of slowdowns.

I've noticed on the SAN that it's showing 100% write cache hit but most of the time less than 50% read cache hit.

So would we benefit from putting in an SSD drive to use as read cache, bearing in mind that the read percentage is around 40% ?
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Dr. KlahnPrincipal Software EngineerCommented:
Will you benefit?  Yes.  With more read cache, no matter where it is located, the read latency will decrease and the physical drives will execute fewer seeks.

Will the benefit affect your operations?  Probably not, if it is already acceptable.  If it were unacceptable, that would be a different situation.

Is it worth the money?  The big question.  Unfortunately, only you can answer that.

Do remember that even SSDs fail. When an SSD used for caching fails, it is a situation far out of the ordinary, it is completely unexpected, and thus it may be difficult to figure out where in the storage subsystem the problem is located.

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devon-ladAuthor Commented:
Hmm...just got a quote back from Dell...£3500 just for the premium feature key.  This is part of the high performance tier feature.  Then there's the drives on top of that.  Might end up paying more than the original price for the SAN!

Just reading the associated documentation for the high performance tier and wondering if we're not getting the best out of the base system.  Generally the SAN hasn't been tweaked out of the box - so this might be the first place to start looking.
devon-ladAuthor Commented:
I've just noticed something I don't quite understand.

The disk latency on a particular VM is jumping up to around 700ms at times.

Yet at the same time the SAN is reporting a maximum latency of 40ms on the virtual disk that the host is using.

Virtual disk is not shared, all VHDs are on the same virtual disk.

Where have those 660ms gone?
devon-ladAuthor Commented:
Ok - I'm going off topic a bit.  Specific question has been answered so I'll move to another post.
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