This is my last post in series on FlashCache testing when the cache is placed on Intel SSD card.
This time I am using tpcc-like workload with 1000 Warehouses ( that gives 100GB of data) on Dell PowerEdge R900 with 32GB of RAM, 22GB allocated for buffer pool and I put 70GB on FlashCache partition ( just to simply test the case when data does not fit into cache partition).
Please note in this configuration the benchmark is very write intensive, and it is not going be easy for FlashCache, as in background it has to write blocks to RAID anyway, and write rate in final place is limited by RAID. So all performance benefits will come from read hits
The full report and results are available on benchmark Wiki
on RAID final result: 2556.592 TpmC
on Intel SSD X25-M: 7084.483 TpmC
on FlashCache with 20% dirty pages: 2632.892 TpmC
on FlashCache with 80% dirty pages: 4468.883 TpmC
So with 20% dirty pages the benefit are really miserable, and it is quite explainable ( see note above about write intensive workload), but really on the graph we can see that probably 2h was not enough to warmup FlashCache enough.
And this is interesting problem with FlashCache what I see. Warmup by simple copying files does not work (you need O_DIRECT with proper blocksize), and you only rely on InnoDB in this case, and it takes about 30min+ to fill FlashCache. Solution there would be PRELOAD TABLE / INDEX, and it is in our roadmap for XtraDB.
With 80% dirty pages the performance gain in 1.7x and it is pretty decent, as you can get it for 500$ ( price of Intel X25-M card) ( for this particular workload, your experience may vary!).
On this stage I consider FlashCache as pretty stable and ready for an evaluation on real workloads ( kudos to Facebook team, they provide really stable release).
I actually did pretty bad test – just turned off power on SSD drive in the middle of tpcc-mysql run,
just SSD power, not whole server. No wonder FlashCache complained on failed writes, and after that I restarted full system. I was expecting that database is going to trash, but after restart FlashCache was able to attach previous cache, and MySQL was able to start and finish crash recovery. I am impressed.
In my next rounds I am looking to run similar benchmarks on FusionIO card.
P.S. And if CentOS team reads this post – please change default IO scheduler from CFQ to Deadline. Seriously, it makes so much damage on performance on servers with IO intensive workloads, so it should be the first action after CentOS installation. And I doubt that there big usage of CentOS on desktop systems anyway.