Monday, 14 May, 2018
Historically, Random I/O Reads were always a major PITA for any OLTP workload.. If Random I/O Writes you could yet "delay" via controller's caches (or any kind of other battery-protected caches -- specially if Writes are coming in bursts), there is no way to "predict" I/O Reads if they are fully Random (so you cannot "cache" or "prefetch" them ahead and have to deliver the data directly from storage, read by read.. -- which is hitting a huge "rotation penalty" on HDD).
Indeed, things changed dramatically since arriving of Flash Storage. You don't need to spend any particular attention if your I/O Reads are Random or Sequential. However, you still need to keep in mind to not hit the overall throughout limit of your Flash Device. As the result, reading by smaller I/O blocks allowing you to do more I/O operations/sec than with bigger blocks.
And what about InnoDB ? -- InnoDB is using by default 16KB page size (so by default all Random I/O Reads are of 16KB) :
And with 16KB Random Reads you definitively will reach your throughput limit sooner than with 8KB or 4KB Reads. Many users are seeing "compression" as the best matching solution here. And, indeed, if you're very lucky and can compress your data, say, by x4 times => you'll Read then only 4KB from storage to deliver 16KB of data. And, yes, you'll be able to deliver x4 times more Reads/sec :
The only problem is that you'll still deliver 16KB of data which you need first to copy to Buffer Pool (BP) to be able then to process the query, and your BP is not unlimited ! ;-)) -- so, in reality, you're able to read data x4 times faster from your storage, but still not able to process them faster due BP size limit :
However, if your rows are well grouped within 16KB page -- you may still win on doing less I/O Reads, but not get any overall QPS performance gain comparing to "non compressed" default case)..
On the same time you have another solution to radically improve your QPS and get a bigger profit of your Flash Device -- you may just use 4KB page size for your whole InnoDB instance, which will allow you within the same I/O throughput level to Read x4 times more and process x4 times more having the same BP size :
Unfortunately, such an approach was possible to use with InnoDB only "partially" until now, because historically every I/O operation in InnoDB was going via a global lock (fil_system mutex) quickly limiting your overall I/O rate. The good news is that since MySQL 8.0 this limitation is finally gone and our I/O scalability moved to the next level ! ;-))
But this all may still remain just a good "theory" if there was no any "practical" result to confirm the claim, right ? -- so, to confirm the gain I'll use IO-bound Sysbench Point-Select workload :
- test case : Sysbench 1.x uniform point-selects
- volume : 50M x 8 tables (400M rows in total, ~100GB)
- BP size : 32GB
NOTE : while there is still a possibility that some data will be re-used from BP, this test scenario is generally saturating any flash device much faster then you can imagine.. -- so, the whole processing here is really IO-bound and IO-driven.
And I'd say the timing was very favorable here to allow us to test it on the latest Intel Optane SSD drive ! -- this device by itself seems to bring the next level of standard to the whole Flash Storage Story ;-)) Here are few "pure IO" test results I've obtained from a single Optane device on my validation tests :
Intel Optane SSD drive :
- singe thread pure Random I/O Reads with 16KB : 65.000 reads/sec (!!), 1000 MB/sec
- avg latency : 0.01ms (!!)
- max throughput : 2000-2500 MB/sec
So, on a pure I/O test on this drive a single thread is already delivering 65K Random Reads/sec of 16KB ! -- but also reaching 1000 MB/sec throughput, means with 2 or 3 threads we're already reaching the max IO rate on this device. However, there is a big difference between "pure IO" test and database "IO-bound workload" -- there is still a lot of processing around every SQL request (not only I/O)..
But "in theory" by using x2 Optane drives together there should be possible to reach :
- with 16KB page size : 260K Random Reads/sec
- with 8KB page : over 500K/sec
- with 4KB page : probably 1M Reads/sec
and as usual, the real answer you obtain only by a real test ;-))
so far :
Sysbench 50Mx8tab IO-bound point-selects
- yes, MySQL 8.0 is showing a real difference here by reaching over 1M IO-bound QPS !!!
- NOTE : the max QPS on MySQL 5.6 and 5.7 is reached with 8KB page size here (become worse on 4KB page due present in these versions fil_system lock contention)..
The results are obtained on the same Skylake server with the same config settings as described before. The only difference is that 50M rows were loaded per table and BP size was reduced to 32GB.
Thank you for using MySQL ! -- stay tuned ;-))
blog comments powered by DisqusNote: if you don't see any "comment" dialog above, try to access this page with another web browser, or google for known issues on your browser and DISQUS..