30 Jan 2012 yeupou   » (Master)

Using RAM for transient data

When a system have lots of I/O, trouble may arise. If an optical hard drive is über-solicited, quite easily you may get many kinds of failures, high CPU load, just because of I/O errors. In such case, using RAM as disk, aka RAM disk, may be a good option, as it allows way more I/O than an optical hard drive. Solid State Drive (SSD) addresses partly this issue, but it seems to, still, have way higher access time and latency than RAM. RAM disk, on the other hand,  is non persistent (unlike SSD, though), quite an annoying drawback so even if you write some scripts to save data, you will loose some in case of power failure.

RAM disk is, actually, especially appropriate for temporary data, like /var/run, /var/lock or /tmp. Linux >= 2.4  supports tmpfs, some kind of RAM disk, that (as far I understand) does not reserve blocks of memory (meaning: it does not matter if you have a big tmpfs, unused memory in the tmpfs will still be available to the whole system).

Most of my computers have more than 1 Gb RAM. And, most of the time, they never use the Swap space. For instance (relevant lines are si and so, as swap in, swap out):

bender:$ vmstat
 procs -----------memory---------- ---swap-- -----io---- -system-- ----cpu----
 r  b   swpd   free   buff  cache   si   so    bi    bo   in   cs us sy id wa
 0  0      0 4146984 674704 1309432    0    0     6     9    3   34  2  1 97  0

nibbler:$ vmstat
procs -----------memory---------- ---swap-- -----io---- -system-- ----cpu----
 r  b   swpd   free   buff  cache   si   so    bi    bo   in   cs us sy id wa
 0  0      0 862044  23944  84088    0    0    10     0   42   22  0  0 99  0

moe:$ vmstat
procs -----------memory---------- ---swap-- -----io---- -system-- ----cpu----
 r  b   swpd   free   buff  cache   si   so    bi    bo   in   cs us sy id wa
 1  0      0 280552 166884 1297376    0    0     7    58   73   12  8  2 90  1

So they are good candidates to use tmpfs whenever possible. Do so with Debian GNU/Linux is fast-forward. Just edit /etc/default/rcS as follows (for /var/run & /var/lock):


and add, in /etc/fstab (for /tmp):

tmpfs             /tmp     tmpfs     defaults    0    0

Next time you boot, diskfree should provide you with something like:

  $ df
Filesystem           1K-blocks      Used Available Use% Mounted on
tmpfs                  1033292         0   1033292   0% /lib/init/rw
varrun                 1033292       648   1032644   1% /var/run
varlock                1033292         0   1033292   0% /var/lock
tmpfs                  1033292         4   1033288   1% /dev/shm
tmpfs                  1033292         0   1033292   0% /tmp

Syndicated 2012-01-30 13:57:36 from # cd /scratch

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