dr strangedrive or: how I learned to stop worrying and love SSDs

I’ve had bad luck with hard drives lately — in the last month or so I’ve lost two of the drives from my desktop PC. Luckily, I’d set up RAID-1 for my Linux install just beforehand, so I didn’t lose anything important (just my Windows drive, hah), but with just one drive left, I needed some kind of replacement.

I could’ve bought another hard drive, but damnit, spinning disks are from the past, and we’re living in the future! Instead, I bought myself a shiny new SSD.

Wolf in mini-sheep’s clothing

To be specific, I got a 256GB Crucial M4 — it’s not the latest and greatest SSD, but it’s been on the market long enough to prove its reliability. It looks so unassuming in its tiny, silent 2.5″ case, but it’s crazy-fast, with read speeds of 450MB/s, write speeds of about 260MB/s (not as fast as some newer drives, but perfectly respectable), and insanely-fast seek times that can make it dozens or even hundreds of times faster than a hard drive in real-world applications.

More than anything else, an SSD makes your PC feel strangely snappy. Boot times and application launch times both benefit hugely — Firefox now takes less than a second to spring to life, even if I’ve only just booted my PC, and staring LibreOffice takes maybe half a second.

Even when attached to a 3.5" bay extender, SSDs look tiny compared to 3.5" hard drives

Even when attached to a 3.5″ bay extender, SSDs look tiny compared to 3.5″ hard drives

To get some numbers, I tested something that’s always been slow on my studio PC: loading large instruments in to LinuxSampler. LS streams most of the sample data on-the-fly, but it still needs to cache the start of each sample in to RAM, and that requires a bunch of seeking. Here you can see the load times for Sampletekk’s 7CG Jr, a 3GB GigaSampler file, and the Salamander Grand Piano, a 1.9GB SFZ, from both my SSD and my old 1TB Seagate Barracuda 7200.12 hard drive — the SSD is about 4-to-6 times faster:

made with ChartBoot

Is flash’s limited lifetime really worth worrying about?

So, SSDs have fantastic performance, and they’re now (relatively) affordable, but I did have one concern: the fact that flash memory cells can only be erased a certain number of times before they wear out. Modern SSDs use techniques like wear-leveling and over-provisioning to minimise writes to each flash cell (this Ars Technica article is a great read if you want to know more), but it’s hard not to think that every byte you write to the drive is hastening its demise.

I worried even more after I ran “iotop” to look at per-process disk usage, and saw that Firefox was writing a lot of data. It writes several things to disk on a regular basis — cached web content, knowing malware/phishing URLs, and crash recovery data — and that can add up to several MB per minute, or several GB per day.

To see if this really was a problem or not, I used iostat to capture per-minute disk usage stats across a typical day. I did all my usual things — I left Firefox, Chrome, Thunderbird, and Steam running the whole time, I spent my work hours working, and then I toyed with some music stuff in the evening. The results are graphed below:

made with ChartBoot

There’s one hefty spike in the evening, when I copied 3.6GB of guitar samples from my hard drive to my SSD (maybe this wasn’t an entirely typical day!), but for the most part, I was writing about 5-15MB per minute to the SSD. The total for the day was 15GB.

That sounds like a lot, but it’s nothing my SSD can’t handle. It’s rated for 72TB of writes over its lifetime, and while that’s an approximate figure, it’s a useful baseline. Over a five-year lifespan, that works out to 40GB of writes a day, or 27.8MB per minute — that’s the red line on the graph above, which was well above my my actual usage for almost the entire day.

When you see a graph like this, it flips your perceptions. If I’m happy to accept a five-year lifespan for my SSD, then every minute I’m not writing 27.8MB to it is flash lifetime that’s going to waste! Smaller SSDs tend to have shorter lifetimes, as do cheaper SSDs, but with typical desktop usage, I don’t think there’s any reason to worry about the life of your SSD, especially if you’re not using your PC 10-12 hours a day or running it 24/7 like I often do.

SSD tuning

There are dozens of SSD tuning guides out there, but most of them spend a lot of time whipping you in to a “don’t write all the things!” frenzy, so instead of linking to one of those, I’ll just reiterate two things that you should do to get the most from your SSD.

The first is to enable TRIM support. This lets the OS tell the SSD when disk blocks are no longer needed (because the files they contained were deleted, for instance); that gives the SSD more spare space to use, which helps reduce drive wear and increases write performance. To enable TRIM, add “discard” to the mount options on each filesystem on your SSD, like so:

/dev/mapper/ssd-ubuntu_root  /  ext4  discard,errors=remount-ro  0  1

IF you’re using LVM, like I am, then you’ll also have to edit the “/etc/lvm/lvm.conf” file, and add the line “issue_discards = 1” to the “devices” section, to make sure that LVM passes the TRIM commands through to the SSD.

The second is to select an appropriate IO scheduler. IO schedulers are bits of code within the Linux kernel that arrange read and write operations in to an appropriate order before they’re sent to the disk. The default scheduler, “CFQ”, is designed to keep for desktop loads on regular hard drives, but its efforts are wasted on SSDs, where seek times are so much lower.

For SSDs, you’re better off with the “deadline” scheduler, which is designed for high throughput on servers, where disks tend to be faster, or you can even use the “noop” scheduler, which does no reordering at all. To set the scheduler on boot, add this to your “/etc/rc.local” file (most Linux distros have one of these):

echo deadline >/sys/block/sda/queue/scheduler

To be honest, the choice of IO scheduler probably won’t make much difference — it just improves performance a little (it won’t have any impact on lifespan), but your SSD is going to be so fast regardless that I doubt you’d ever notice. It’s an easy fix, though, so it’s worth the 10 seconds it’ll take to perform.

So go forth, buy an SSD, make a couple of minor tweaks, and then don’t be afraid to enjoy it!

3 thoughts on “dr strangedrive or: how I learned to stop worrying and love SSDs

  1. Pingback: dr strangedrive redux: should you swap on an SSD? | woo, tangent

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