Is it a good idea to mount /var in memory?

Question

Re this answer: https://raspberrypi.stackexchange.com/a/867/339

It is suggested that /var be mounted in memory, so to avoid causing your root partition to fill accidentally.

I've got a couple of questions about this, one particularly RPi related:

1: In the case of logging, are these logs then available to view? - Say the machine crashes or whatever - can you get back to the logs to see what went wrong.

2: In the specific case of the RPi with fairly limited RAM available, is doing this a good idea? - if there's a risk of filling a disk is there not also a problem with running out of memory?

score 7 · Accepted Answer · answered Jul 02 '12 at 08:41

7

You have to evaluate your particular use case. Normally, you would mount a filesystem in RAM for speed.

1: In the case of logging, are these logs then available to view? - Say the machine crashes or whatever - can you get back to the logs to see what went wrong.

If the RPi crashes and you have to restart it, the logs will not be available.

2: In the specific case of the RPi with fairly limited RAM available, is doing this a good idea? - if there's a risk of filling a disk is there not also a problem with running out of memory?

I agree that, in general, RAM is too low to waste some on a filesystem. You do, of course, limit the size of the filesystem.

Alternative

Create another partition on your SD card, so it can fill it up without filling up the entire card.

answered Jul 02 '12 at 08:41

Alex Chamberlain

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If these are many small writes, is it better to have /var off the SD card where possible? (for SD lifetime) – Jon Egerton Jul 02 '12 at 08:56
I guess, but you should probably back it up onto the SD card fairly regularly. – Alex Chamberlain Jul 02 '12 at 09:04
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-1; Mounting a filesystem in ram is not just for speed nor is the most frequent use-case for speed. – user606723 Jul 02 '12 at 12:31
@user606723 Ok, what are the frequent use cases then? – Alex Chamberlain Jul 02 '12 at 12:35
@AlexChamberlain, /tmp in almost all modern Linux distros – user606723 Jul 02 '12 at 12:40
@user606723 Why was /tmp put into RAM? – Alex Chamberlain Jul 02 '12 at 12:42
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It's a place to put files that are non-volatile and can be deleted when necessary. It's certainly not for speed because there is nothing ensuring /tmp is mounted in ram; In fact, many "Production" environments put these partitions on hard drives "just in case" they may need such information for future use. – user606723 Jul 02 '12 at 12:45
let us [continue this discussion in chat](http://chat.stackexchange.com/rooms/3955/discussion-between-alex-chamberlain-and-user606723) – Alex Chamberlain Jul 02 '12 at 12:46
s/non-volatile/volatile/ – user606723 Jul 02 '12 at 12:50

score 5 · Answer 2 · edited Feb 17 '17 at 16:40

Mounting a filesystem in RAM is not just done for speed. The most common case of this is with the average /tmp directory, which is mounted via tmpfs on almost all modern Linux distributions

In fact, instead of mounting /var in RAM, you should consider symlinking the appropriate subdirectories to /tmp

This is what many embedded Linux distributions do (OpenWRT, etc.). Although, keep in mind they are meant to run a much 'smaller' hardware: 8MB flash 32MB RAM

1: In the case of logging, are these logs then available to view? - Say the machine crashes or whatever - can you get back to the logs to see what went wrong.

Of course, you will lose these logs on restart. If you're concerned about this, you could consider a remote syslogd

2: In the specific case of the RPi with fairly limited RAM available, is doing this a good idea? - if there's a risk of filling a disk is there not also a problem with running out of memory?

The amount of logs that will get generated is not as large as you may think. It would be easy to keep them in memory granted the machine isn't running for years. (And then you'd simply need to clean them as you would anyway). That said, this also means the amount of data recorded to the sdcard isn't really that significant either.

Is it a good idea to mount /var in memory?

2 Answers2

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