README.md

This is a set of benchmarking utilities for measuring the latencies of disks (mostly interesting for SSDs).

The lowest level tool is the bench program. This is a microbenchmark that executes repeated writes and fsyncs of the requested size. To build it and see its usage, run:

make bench
./bench --help

A possibly outdated version of that message is:

Time how long it takes to append into a file and fdatasync it.
The results are printed in seconds total for all the writes.

Usage: ./bench [options]
  --count=NUM      Number of sequential appends to measure [default 1000]
  --direct=yes|no  Whether to use O_DIRECT [default no]
  --file=NAME      File to create/truncate and write into [default bench.dat]
  --help           Print this help message and exit
  --offset=BYTES   Number of bytes to skip at start of file [default 0]
  --size=BYTES     Number of bytes to append in each iteration [default 1]

While the bench program can be useful on its own, you'll probably want to run the benchmark with a bunch of different paramters. To do so automatically, you can use runner.sh. First, you'll need to create a file inside the machines/ directory describing the machine you want to test on and its disks. The name of the file can be any name you want for the machine, and the file itself is interpreted by bash.

Here's an example for testing against a single disk on localhost:

disks="somedisk:sda:/tmp"
rootcmd () {
    sudo $*
}
cmd () {
    $*
}
sendfile () {
    cp $1 ~/
}

And here's a more complex example for testing against two disks on a remote host:

disks="x25-m:sda:/home/ongardie 530:sdb:/home/ongardie/sdb1/tmp"
rootcmd () {
    ssh flygecko sudo $*
}
cmd () {
    ssh flygecko $*
}
sendfile () {
    scp $1 flygecko:
}

There's four things you need to define in each machine file:

• disks is a string listing out the disks attached to the system. Each disk is separated with a space. There are three components to each disk, separated by colons: the disk's friendly name, the disk's device name within /dev/, and the directory in which the benchmark should write its file.
• rootcmd is a function that executes a command as root. This is used for changing device settings with hdparm.
• cmd is a function that executes a command as a normal user. This is used for compiling and running the bench program.
• sendfile is a function that copies the given file to the machine's home directory.

You'll need gcc and hdparm installed on every remote machine.

Once you've got your machine description file in place, you should review the options at the top of the runner.sh script. It's rather thorough out of the box, and you might want to cut back on the number of configurations. When you're ready, run:

./runner.sh

This script produces a file called results.csv that looks something like this:

machine,disk,writecache,size,offset,direct,count,seconds
flygecko,x25-m,yes,1,0,no,100,0.033465235
flygecko,x25-m,yes,1,512,no,100,0.029445542
flygecko,x25-m,yes,2,0,no,100,0.034066043
flygecko,x25-m,yes,2,512,no,100,0.032349597
flygecko,x25-m,yes,4,0,no,100,0.030776787
flygecko,x25-m,yes,4,512,no,100,0.033931288
...

You'll probbaly want to tail -f results.csv while runner.sh is working so that you feel better about progress.

Because the results.csv file gets to be large, it's probably necessary to post-process it somehow for human consumption. The script post.R loads in the results.csv file, summarizes the data a bit, and graphs the results. To invoke it, run:

R -e "source('post.R'); ggsave('results.svg', g, width=10, height=7)"

If you know R and ggplot2, you'll find this to be a good starting point. If you don't, your mileage may vary with the default graph. A sample graph is included in results.sample.svg:

.

That's everything you'll find here for now. Go test your disks! Pull requests are welcome.