Mongoperf

Starting with version 2.1, MongoDB has a new utility called Mongoperf. Mongoperf is a great little utility for quickly testing the I/O performance of your system. I thought I would go over it a little bit.

Mongoperf can be found in the bin directory of your 2.1+ MongoDB distribution. The utility is invoked with options to tell it how to perform a sample I/O run, and it gives output indicating the performance of your disk I/O subsystem. The utility generates random I/O over a single file. One great aspect of this utility is it accesses the I/O subsystem very much like MongoDB itself does. It uses the same memory mapped files interface just like MongoDB itself. Running the utility with –help outputs the various options for running. The utility takes a JSON document as input. Typically I like to store the various options in a .js file and emit that into stdin of the utility as such:

cat mongoperf_settings.js | mongoperf

The most powerful usage of this tool is to bypass the Linux page cache, and perform Direct I/O. Running in this manner shows true gauge on how fast your storage subsystem is. This is done via the {mmf:false} assignment in the config document. When mmf is false, then Mongoperf performs direct I/O via the O_DIRECT flag. So no system memory is used to cache file buffers. That said, anything below the linux filesystem could be cached, including controller and drive caches. For example a config that looks like this:

{ recSizeKB: 8, nThreads: 12, fileSizeMB: 10000, r: true, mmf: false }

The output of the utility shows the throughput as the threads ramp up to the nThreads limit you specify and then will just run at that level of concurrency forever. The output is in Disk Operations per Second. For example:

new thread, total running : 1
read:1 write:0
4759 ops/sec 18 MB/sec
4752 ops/sec 18 MB/sec
4760 ops/sec 18 MB/sec
4758 ops/sec 18 MB/sec
4752 ops/sec 18 MB/sec
4754 ops/sec 18 MB/sec
4758 ops/sec 18 MB/sec
4755 ops/sec 18 MB/sec
new thread, total running : 2
9048 ops/sec 35 MB/sec
9039 ops/sec 35 MB/sec
9056 ops/sec 35 MB/sec
9029 ops/sec 35 MB/sec
9047 ops/sec 35 MB/sec
9072 ops/sec 35 MB/sec
9040 ops/sec 35 MB/sec
9042 ops/sec 35 MB/sec

There are a couple things to note about this utility. For one, be sure to specify a reasonably large file for fileSizeMB. Maybe like 10000. This ensures you are wider than a single stripe if using RAID, this also ensures that you have enough data on disk that random I/O is truly random. Let the test run long enough that the system normalizes (caches get populated, etc). Try for 3-5 minutes or so as a general guideline. Also try different nThreads values to get a feel for the level of disk concurrency your system can tolerate. Also, another great test is to test the read and write options together, and separate to get a feel for how well your system does on each on of these types of workload. For instance, you can test the write cache on a controller quite effectively using this approach.

In the future I think having an option for creating multiple files to I/O against would be a great option and allow for a bit more realistic workload. Perhaps an option to output the results in BSON/JSON would be cool too.