Comments on: FlashCache: tpcc workload

By: Vadim

Vadim — Thu, 21 Apr 2011 22:02:12 +0000

Mark,

TRIM probably won’t help there.
What I mean is that we can expect general performance degradation due fact that card will be fully filled.

By: Mark Callaghan

Mark Callaghan — Thu, 21 Apr 2011 14:05:16 +0000

What is there to TRIM during this test? There are lots of old block versions to collect, but the flash/ssd device will already know that. Are any files being deleted?

Is the tpcc-mysql workload uniform (all rows or disk pages equally likely to be used)? If yes, then flashcache mostly acts like a huge write cache and might not be worth the extra cost. But many workloads are highly skewed in production and for that case flashcache can make a big difference.

By: Vadim Tkachenko

Vadim Tkachenko — Thu, 21 Apr 2011 03:17:40 +0000

Tom,

It is hard to say. If you have very write intensive workload, maybe FlashCache will not work as expected in any case.
For FusionIO you may look into their DirectCache cache instead of FlashCache.
Virident tachIOn cards (only SLC now) shows acceptable performance with full disk, so they may work better with FlashCache.

By: Tom

Tom — Thu, 21 Apr 2011 02:51:14 +0000

Vadim,

Thanks for the input!

Do you think it is only making sense to use expensive SSD or flash-drive such as X25E or fusion io drive with FlashCache due to the SSD write performance limitation?

Would SSD made of MLC be problematic with FlashCache as a server cache after the cache becomes full? Therefore, in a real production environment, we should always spend big to get SLC SSD?

Thanks,
Tom

By: Vadim

Vadim — Thu, 21 Apr 2011 01:36:16 +0000

Tom,

Yes, I think the write performance will be problematic with full SSD, but I did not test that
specifically.

By: Tom

Tom — Wed, 20 Apr 2011 15:45:34 +0000

Vadim,

Thanks for the benchmark!

If we write like 1TB data to flashcache, when the SSD is full, does the write performance decrease to below 300 IOPS since no TRIM is available in flashcache?

Thanks!

Tom

By: Brian Lovett

Brian Lovett — Sat, 29 May 2010 15:56:46 +0000

This is interesting. Just yesterday I noticed that our I/O is really bottlenecking our system and am looking into a pair of the intel SSD’s, but the SLC version as our dedicated server host offers them at a decent price (monthly). I was going to use one of them for our sphinx indexes and have been debating using the second one for flash cache or to house MySQL directly. Our data set is about 30GB, but we only have 12GB of memory on the machine currently. It looks like using the second drive as a cache might be the way to go. Was the setup difficult on CentOS? More reading for me

By: Vadim

Vadim — Thu, 27 May 2010 00:18:47 +0000

Andy,

There is BBU on RAID, but no BBU on SSD, it is directly attached and it has write cache enabled.

I did not test SSD with BBU, so I can’t provide good feedback there.
Disabling write cache on SSD really affects write performance a lot.

The reason I recommend deadline is that it shows almost the same results with noop on my RAID system,
but noop really makes sense if your IO subsystem can manage IO requests properly ( Perc6/i does that),
otherwise better to delegate this work to OS ( deadline scheduler).

By: Andy

Andy — Wed, 26 May 2010 21:43:21 +0000

Vadim,

- In your test did you use BBU for the RAID 10? Any BBU for the SSD?

- If I put my data on SSD, does it make much difference whether I use BBU or not?

- Any reason why you recommend deadline instead of noop?

By: FractalizeR

FractalizeR — Wed, 26 May 2010 16:34:28 +0000

Aha, now I see Thanks