Which root paths should reside on fast media?

Question

Which paths under / should be mounted on fast media, and which can be mounted on slower media without significantly affecting system performance?

My guess:

Fast

• Partition: swap (2G)
• Partition: root
  • /etc (16M)
  • /bin (8M)
  • /var (500M)

Slow

• Partition: boot
• /home
• /media
• /mnt
• /root
• /usr (5G)

Reason

I have two older laptops that only take a 1.8" PATA drive. The laptops are ultra-portable and still performing their function well so I don't wish to "upgrade" (it's hard to get a more performant ultra-portable). The HDD in one laptop is failing. Sourcing 1.8" PATA drives is hard enough now, but will surely get more so. I opted to buy a PATA-CF card adapter that allows replacing the HDD with a master (and optionally) a slave CF card. High bandwidth CF cards are expensive; I'd like to have a limited capacity high bandwidth card to maintain system performance, and a lower bandwidth but higher capacity card for everything else. So - which paths do I mount where?

Context

FWIW, I run Gentoo (several years experience). These laptops are older Pentium-M so on 32 bit x86 architecture.

Answer 1

The bits that will get lots of disk write traffic are /tmp and /var. Binaries directories will get some read traffic, although the actual working set is relatively small and will mostly be cached on a machine more than a few hundred MB of RAM.

Historically, unix systems were built with a mixture of fast and slow disks due to peculiarities of the DEC hardware of the era. The original layout was based on fast disks for root (including /bin) and swap and a slower disk for /usr. However, these machines had much less RAM than a modern computer, so RAM caching will pad the binaries traffic as it is mostly read-only.

In your case, I'd suggest that you keep it simple and put everything but /home on the fast disk. You shouldn't actually need more than a few GB for this and 8-16GB high-speed CF cards are quite cheap. Get a bigger disk for /home.

Answer 2

It all depends on mainly two things:

1. What programs are typically run on the computer

2. What aspect of performance you value the most

Below, I'll assume that you use the computers as a more or less standard desktops and that the main aspect of performance you could (and probably want to) gain is program startup times.

Looking at your fast/slow concepts, I'd suggest following changes:

• /etc contains files of high importance, but relatively small in size. This does not necessarily need to placed on a fast medium.

• /lib and /usr/lib are the probably the most important parts of the system tree when it comes to startup times - have a look at your lsof and do a grep -cF '/lib/' at a moment of typical system usage. Compare to grepping for \(/bin\|/sbin\).

So I'd rather put /usr/lib on a faster partition. The problem would of course be its size. You probably wouldn't try to put the whole /usr on a faster partition - better make /usr/lib a symlink or a bind mountpoint. If the size of it is still too big for your setup, you can go one level deeper and create symlinks only for the subtrees of most used libraries - or the contrary - place the main /usr/lib on fast medium and clean away the less used sub-directories, turning them into symlinks. For instance I would link away programming stuff like gcc, perl or python.

Apart from that, as you wrote, /var should certainly be worth putting on a fast meduim. However beware that you'll need a good CF card that will not die too soon because of excessive writing!