In my previous article I got awful results for MongoDb and some "proofs" led me to wrong away. Actually not all of them were wrong, but before let's deal with simplier problem.
My benchmark results were 29 inserts per second, 34 ms per insert. I thought it was due to HDD seeks, because MongoDb updates two files per write. But when I ran same test on SSD I got same results. Something blocks my requests! And here it is:
I was surprised that write to journal is also delayed, looks like guys from 10gen try to group some disk operations together. The problem is that every write is delayed, unconditionally! I ran MongoDb without specifying journalCommitInterval, so in my case delay is oneThird = (100 / 3) + 1 = 34!!! My every insert operation is delayed by 34 ms! Yes, sir, my benchmark confirms it!
After reading documentation I've found out that smallest valid journalCommitInterval is 2 which gives oneThird = (2 / 3) + 1 = 1 ms. I ran MongoDb with this param value expecting to get something around 1K inserts per second. But I got only 700 inserts per second. Not that much... Learned from my previous mistakes I ran same benchmark at my SSD and got around 980. Much closer to that I expected keeping in mind 1 mspenalty delay.
There is something fishy why MS SQL doesn't update 'last write time' but MongoDb does...
UPDATED: how to write to journal without additional HDD seeks
My benchmark results were 29 inserts per second, 34 ms per insert. I thought it was due to HDD seeks, because MongoDb updates two files per write. But when I ran same test on SSD I got same results. Something blocks my requests! And here it is:
I was surprised that write to journal is also delayed, looks like guys from 10gen try to group some disk operations together. The problem is that every write is delayed, unconditionally! I ran MongoDb without specifying journalCommitInterval, so in my case delay is oneThird = (100 / 3) + 1 = 34!!! My every insert operation is delayed by 34 ms! Yes, sir, my benchmark confirms it!
After reading documentation I've found out that smallest valid journalCommitInterval is 2 which gives oneThird = (2 / 3) + 1 = 1 ms. I ran MongoDb with this param value expecting to get something around 1K inserts per second. But I got only 700 inserts per second. Not that much... Learned from my previous mistakes I ran same benchmark at my SSD and got around 980. Much closer to that I expected keeping in mind 1 ms
There is something fishy why MS SQL doesn't update 'last write time' but MongoDb does...
UPDATED: how to write to journal without additional HDD seeks
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