Adaptive Flash Cache Stats

jonesy5090 · **Joined:** Mon Nov 10, 2014 8:33 am **Posts:** 3

Hi folks,

Just wondering if anyone has had a play with Adaptive Flash Cache? I've started it in simulator mode with 128GB as a starting point and I'm seeing an average of just over 2% in additional read hits. At the moment it doesn't seem worth enabling, I think we're better off using the SSD space for AO. Would like to know your thoughts.

If you want to enable AFC in simulator mode the commands are:

Code:

createflashcache –sim 128g

To enable AFC, followed by:

Code:

setflashcache enable vvset:flashcachetest

Where flashcachetest is the name of your VV Set, I've only enabled it for our VMware datastores at the moment.

Cheers, Gaz.

hdtvguy · **Joined:** Sun Jul 29, 2012 9:30 am **Posts:** 576

Not sure how big your array is, but 128GB sounds small. a 7400 can go to I believe 1.5TB per node pair?

jonesy5090 · **Joined:** Mon Nov 10, 2014 8:33 am **Posts:** 3

The VV set that it's enabled for is 15TB in size so it's not huge, they're enabled for AO as well so most of the hot blocks are probably on SSD already.

Cleanur · **Joined:** Wed Aug 07, 2013 3:22 pm **Posts:** 254

Be careful about reaching conclusions based on artificial benchmarks as this probably isn't representative of a real data access pattern.

Being able to gauge the benefits is precisely why the AFC simulator exists, for many Customers especially with smaller active data sets the internal Controller DRAM coupled with wide striping is more than sufficient. It's one of those YMMV, if you need it you'll definitely see big benefits, not only will it accelerate front end I/O but it will also unload the backend disk. For many users, trying to constrain this to a single VVSET may be a mistake, the unloading of the backend (assuming you have enough control memory and SSD) means your probably better off applying this to the system as a whole, after all a rising tide raises all boats.

Schmoog · **Joined:** Wed Oct 30, 2013 2:30 pm **Posts:** 242

Very true.

Also bear in mind that flash cache only works well for certain workloads. It took hp a very long time to enable it, and they kept saying that most of their customers didn't need it.

Even emc recommends that the workload be evaluated before turning on flash cache because for a lot of workloads it does very little

jonesy5090 · **Joined:** Mon Nov 10, 2014 8:33 am **Posts:** 3

tbh our vSphere data stores and SQL DBs are the only workloads we'd be interested in enabling for AFC and most of their active data has already been punted to SSD by AO. I was surprised at how little difference it's made considering the difference in size between DRAM cache and Flash Cache.... I guess it just goes to show that the majority of I/Os occur in across a small set of data, our cache hits are 88% regardless:

Code:

13:46:17 11/11/2014 ------- Current --------    -------- Total ---------
                              CMP  FMP Total              CMP  FMP Total
    Node Type       Accesses Hit% Hit%  Hit%    Accesses Hit% Hit%  Hit%
       0 Read           2657   88    1    89        2657   88    1    89
       0 Write          8746    8    0     8        8746    8    0     8
       1 Read           3290   86    3    89        3290   86    3    89
       1 Write          9595    8    0     8        9595    8    0     8

                             Internal Flashcache Activity
                    ----- Current ------     ------- Total --------
Node Type           Accesses  IO/s  MB/s         Accesses IO/s MB/s
   0 Read Back            30     0     0               30    0    0
   0 Destaged Write       99     0     0               99    0    0
   1 Read Back            91     0     0               91    0    0
   1 Destaged Write      128     0     0              128    0    0

     -------------------- FMP Queue Statistics --------------------
Node Dormant    Cold   Norm   Warm   Hot Destage Read Flush WrtBack
   0  416747 2300241 569355 881664 26297       0    0     0       0
   1  334550 2226102 705316 845437 82899       0    0     0       0

     ------------------- CMP Queue Statistics --------------------
Node  Free  Clean Write1 WriteN WrtSched Writing DcowPend DcowProc
   0 18213 161198   6187    304     2949       0        0        3
   1 19192 160172   6867    433     1579      97        0        0

bajorgensen · **Joined:** Wed May 07, 2014 10:29 am **Posts:** 142

If your data density "hotness" is so small it fits in memory cache,
AFC will not improve that much.
You get diminishing returns the higher your hit rate...

My understanding is that AFP is quite selective in what to put into cache as well.
Only random IO below 64kB?

hdtvguy · **Joined:** Sun Jul 29, 2012 9:30 am **Posts:** 576

I wish 3par would have offered cache upgrades over flash cache. I hear the newer V series comes with double the cache the older units came with, but there is no customer upgrade path. I am also interested to test AFC in our DR site as we get killed with performance when we bring up DR since AO has pushed everything down to NL, also eager for write based AFC which I am not sure when that is coming.

ailean · **Joined:** Wed Nov 09, 2011 12:01 pm **Posts:** 392

I imagine you can buy the DIMMs to upgrade, the issue that I'm aware of is that it can't be done online as node pairs must have the same amount of cache for the redundancy code etc. Hence it would require a shutdown to install. Not sure if the rest of the controller is the same but again if it needed controller swaps it would be a shutdown (not something we expect to do in the life of the arrays

).

Cleanur · **Joined:** Wed Aug 07, 2013 3:22 pm **Posts:** 254

There are other considerations as well around pinned data space, hold up power and the internal node SSD disk etc all related to power fail and cache destage to disk etc. So it's not quite as simple as adding more DRAM to a node, but flashcache gives you pretty much the same if not more benefits with a larger capacity and lower cost without any of the potential disruptions. Also keep in mind this isn't the end state for Adaptive Flash Cache, there's a reason it's configured using Raid10 LD's.

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