/usr on an SD card

SD cards do not have very good read speed, and usually have awful write speed. However, they sort of make up for that (under some conditions anyway) by having no seek delays. Because my netbook has a 5400 RPM hard drive and shows annoying levels of I/O latency (and throughput is not a big deal), I've installed Linux with an SD card mounted as /usr.

OS: SalixOS 13.37 Mate
Partition layout: 2 GB /dev/sda1 swap, 158 GB /dev/sda2 / as ext4, 4 GB /dev/sdb1 /usr as ext4 (though I use a filesystem label so as to avoid depending on device order).

There are several advantages of this over other partition layouts with an SD card IMO:
- /usr is where most desktop binaries are, so it will probably benefit most from the low seek times.
- Not having /var, etc. on the SD card means you save a lot of space.
- Since system binaries are (usually) not stored in /usr, you can (where applicable) boot without a complicated initrd setup.
- /usr doesn't get written to that much, so this layout should probably reduce wear on the SD card.

Does it work? Well... Kind of.
- Installation is very, very slow, taking about twice as long as when using just the hard drive.
- Boot is noticeably faster, by maybe 8-10 seconds in wall clock terms.
- Some applications launch faster, a few launch slower, most launch in the same amount of time (give or take half a second).
- If there's any impact on overall responsiveness, for better or worse, I haven't noticed it yet.

Is it worth the bother? Probably not. But if you have the spare time, a spare flash drive, and a pokey low-end computer, you could try it anyway...

Oh, one other thing - my particular netbook uses a USB card reader, instead of the usual MMC type. I don't know if this is significant when it comes to performance.

Addendum: OTOH, putting /usr on a flash device makes a huge difference for the better in KDE 4 start times and responsiveness.