Starbeamrainbowlabs

Stardust
Blog

Read / Write Disk Performance Testing in Bash

Recently I needed to quickly (and non-destructively) test the read / write performance of a flash drive of mine. Naturally, I turned my attention to my terminal. This post is me documenting what I did so that I can remember for next time :P

Firstly, to test the speed of a disk, we need some data to test with. Since lots of small files will inevitably cause slowdowns due to the overhead of writing the file metadata and inode information to the superblock, it makes the most sense to use one gigantic file rather than tons of small ones. Here's what I did to generate a 1 Gigabyte file filled with zeroes:

dd if=/dev/zero of=/tmp/testfile.bin bs=1M count=1024

Cool. Next, we need to copy it to the target disk and measure the time it took. Then, since we know the size of the file (1073741824 bytes, to be exact), we can calculate the speed at which the copy took place. Here's my first attempt:

time dd if=/tmp/testfile.bin >testfile.bin

If you run this, you might find that it doesn't take it very long at all, and you get a speed of something like ~250MiB / sec! While impressive, I seriously doubt that my flash drive has that kind of speed behind it. Typically, flash memory takes longer to write to and read from - and I'm pretty sure that it can't read from it that fast either. So what's going on?

Well, it turns out that Linux is caching the disk write operations in a buffer, and then doing them in the background for us. Whilst fine for ordinary operation, this doesn't give us an accurate representation of how fast it's actually writing to the disk. Thankfully, there's something we can do about this: Use the sync command. sync will flush all cached write operations to disk for us, giving us the actual time it took to write the 1 GiB file to disk. Here's the altered command:

sync;
time sh -c 'dd if=/tmp/testfile.bin >testfile.bin; sync'

Very cool! Now, we can just take the time it took and do some simple maths to calculate the write speed of our disk. What about the read speed though? Well, to test that, we'll first need to clear out the page cache - another one of Linux's (many) caches that holds portions of files that have recently been accessed for faster retrieval - because as before, we're not interested in the speed of the cache! Here's how to do that:

echo 1 | sudo tee /proc/sys/vm/drop_caches

With the correct cache cleared, we can test the read speed accurately. Here's how I did it:

time dd if=testfile.bin of=/dev/null

Fairly simple, right? At a later date I might figure out a way of automating this, but for the occasional use now and again this works just fine :)

Found this useful? Got a better way of doing it? Want to say hi? Post in the comments below!

Tag Cloud

3d account algorithms android announcement architecture archives arduino artificial intelligence artix assembly async audio bash batch blog bookmarklet booting c sharp c++ challenge chrome os code codepen coding conundrums coding conundrums evolved command line compilers compiling compression css dailyprogrammer debugging demystification distributed computing documentation downtime electronics email embedded systems encryption es6 features event experiment external first impressions future game github github gist gitlab graphics hardware hardware meetup holiday holidays html html5 html5 canvas infrastructure interfaces internet io.js jabber jam javascript js bin labs learning library linux low level lua maintenance manjaro network networking nibriboard node.js operating systems performance photos php pixelbot portable privacy problem solving programming problems projects prolog protocol protocols pseudo 3d python reddit redis reference release releases resource review rust searching secrets security series list server software sorting source code control statistics storage svg technical terminal textures three thing game three.js tool tutorial tutorials twitter ubuntu university update updates upgrade version control virtual reality virtualisation visual web website windows windows 10 xmpp xslt

Archive

Art by Mythdael