Sunday, October 26, 2008

SSDs and the return of the root partition

Back in the days when Unix was new to the world computers came with two sorts of mass storage: fast-small-expensive, and slow-big-cheap. A minicomputer typically had one of each, so that frequently-used files could be kept in the fast unit while everything else was kept on the slow one.

Unix was designed with this in mind, which is why to this day most programs on Unix and Linux are found in one of two directories: /bin holds the frequently used ones, and /usr/bin holds the less frequently used ones. A similar arrangement applies to libraries, which are held in /lib and /usr/lib. The idea was that the "root" partition was put on the fast small device, and the "user" partition went on the big one. This led to some other restrictions as well; /bin and /lib together have to have all the software needed for a minimally functioning system, because until all that stuff is going the system can't mount /usr to get at the rest of it.

All that became irrelevant in the age of the Linux PC because the entire system hung off one physical disk. But maybe that is about to change. Solid State Disks (SSDs) are entering the disk world. They have zero mechanical latency and their read speed is blisteringly fast. Write speed is more variable, but the new Intel product is reported to be very fast. It is also small (80GB) and expensive ($600).

So it looks like the time has once again arrived for keeping the commonly used programs and libraries on a small fast disk and everything else on something big and slow. The original root versus /usr division doesn't work for this of course: people want the system to fire up X windows, KDE or Gnome and a bunch of apps, not just a shell prompt. But all of those things will fit very nicely onto one of these SSDs. Better yet, the user files have now been shifted out from /usr (from which it got its name) and put in /home. So this needs zero surgery to the standard Linux layout.

For the average user $600 is a bit steep though. What is actually needed here is something smaller and cheaper. I run a well-populated version of Fedora, and my root partition with all the Fedora packages is under 10GB. So I while I can't imagine blowing $600 (or whatever that is in £s these days) on 80GB of SSD, I could certainly imagine putting a 10GB version in for $100. That would hold the Linux kernel, X Windows and all the apps, with enough room left over for /etc. That would increase the speed of booting up, logging in and starting applications without greatly increasing the cost of the system, especially since that often involves reading lots of different files scattered around the disk. Even better, most of this stuff is only written rarely, and then mostly in the background by system updates. So the current poor write performance of most SSDs doesn't matter.

(Update: I have of course forgotten that by default Unix and Linux keep track of the access time (atime) of files, which means that every time you read something a little metadata write happens. So write performance does matter here).

Looking around, the nearest product to this specification seems to be the OCZ 32GB unit, which retails for a bit over £100 (say, around $170). Thats bigger and more expensive than what I want, but hey, Moore's Law seems to be working so I'll just wait a bit. I also note that all these products seem to be targetted at the laptop market, which makes sense given SSDs other advantages of low power consumption and physical robustness. Maybe they should start thinking about the market for hybrid desktop systems as well.


Robin said...

Write performance isn't an issue because you can just turn off atimes with the "noatime" mount option. Almost no programs need atimes.

maltem said...

I've been told that usr is short for unix system resources, rather than user. Not sure what the real truth is.

Anonymous said...

I hate to be contrary but the reason some binaries are in /bin and some are in /usr/bin is to do with the order in which partitions are mounted at various stages in the boot process (the same applies to /sbin and /usr/bin). The root partition is mounted by the boot loader and all the others are mounted by the OS so the binaries to mount the other discs need to be on the root partition, hence /bin /sbin. Of course, given that things to breeak there are always enough tools on the root parition to try and fix any problems.