I have mentioned a few times that Innodb caches data in pages and even if
you have a working set consisting of relatively few rows your working set
in terms of pages can be rather large.
Now I came to do a little benchmark to show it in practice. I’m using standard “sbtest” with 10mil rows with data file of 2247098368 which gives us 224 bytes of gross storage per row, including all overhead etc. Actual row
size in this table is smaller but lets use this number for our math. For benchmark I’m using set number of random IDs which are repeatedly selected in random order, which would illustrate the data set with
some randomly distributed “hot” rows. I read every row in the set once before timing, so when there is enough memory to cache every single row there should not be any disk reads in benchmark run itself.
Testing 100 out of 10000000 keys 24.79 seconds, 4034.53 lookups per second Testing 200 out of 10000000 keys 25.66 seconds, 3896.96 lookups per second Testing 400 out of 10000000 keys 25.03 seconds, 3995.65 lookups per second Testing 800 out of 10000000 keys 24.40 seconds, 4097.73 lookups per second Testing 1600 out of 10000000 keys 24.83 seconds, 4026.59 lookups per second Testing 3200 out of 10000000 keys 25.47 seconds, 3926.65 lookups per second Testing 6400 out of 10000000 keys 26.51 seconds, 3771.73 lookups per second Testing 12800 out of 10000000 keys 386.20 seconds, 258.94 lookups per second Testing 25600 out of 10000000 keys 640.12 seconds, 156.22 lookups per second Testing 51200 out of 10000000 keys 775.38 seconds, 128.97 lookups per second Testing 102400 out of 10000000 keys 841.65 seconds, 118.81 lookups per second
As we see in this case database can really fit only somewhere between
6400 and 12800 different rows which is about 1/50 of “projected size”.
This number is very close to what I would have estimated –
With 224 bytes per row we have some 70 rows per page so with random distribution you would expect up to 70 times data which have to be fetched to the database than you need.
I’m wondering if any over storage engine can show better results in
such benchmark. Falcon with plans for row cache would fair better, so I
would expect better results with PBXT. I also should check with
smaller page sizes available in Percona Server and my expectation is with 4K page size I can fit 4x more distinct rows in my cache.