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I'm currently facing a problem on a linux box where as root I have commands returning error because inotify watch limit has been reached.

# tail -f /var/log/messages
[...]
tail: cannot watch '/var/log/messages': No space left on device
# inotifywatch -v /var/log/messages
Establishing watches...
Failed to watch /var/log/messages; upper limit on inotify watches reached!
Please increase the amount of inotify watches allowed per user via '/proc/sys/fs/inotify/max_user_watches'.` 

I googled a bit and every solution I found is to increase the limit with:

sudo sysctl fs.inotify.max_user_watches=<some random high number>

But I was unable to find any information of the consequences of raising that value. I guess the default kernel value was set for a reason but it seems to be inadequate for particular usages. (e.g., when using Dropbox with a large number of folder, or software that monitors a lot of files)

So here are my questions:

  • Is it safe to raise that value and what would be the consequences of a too high value?
  • Is there a way to find out what are the currently set watches and which process set them to be able to determine if the reached limit is not caused by a faulty software?
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1 Answer 1

up vote 42 down vote accepted

Is it safe to raise that value and what would be the consequences of a too high value?

Yes, it's safe to raise that value and below are the possible costs [source]:

  • Each used inotify watch takes up 540 bytes (32-bit system), or 1 kB (double - on 64-bit) [sources: 1, 2]
  • This comes out of kernel memory, which is unswappable.
  • Assuming you set the max at 524288 and all were used (improbable), you'd be using approximately 256MB/512MB of 32-bit/64-bit kernel memory.
    • Note that your application will also use additional memory to keep track of the inotify handles, file/directory paths, etc. -- how much depends on its design.

To check the max number of inotify watches:

cat /proc/sys/fs/inotify/max_user_watches

To set max number of inotify watches

Temporarily:

  • Run sudo sysctl fs.inotify.max_user_watches= with your preferred value at the end.

Permanently:

  • Replace the value within the /proc/sys/fs/inotify/max_user_watches file with your own, i.e echo 524288 | sudo tee -a /proc/sys/fs/inotify/max_user_watches.

Check to see if the max number of inotify watches have been reached:

Use tail with the -f (follow) option on any old file, e.g. tail -f /var/log/dmesg: - If all is well, it will show the last 10 lines and pause; abort with Ctrl-C - If you are out of watches, it will fail with this somewhat cryptic error:

tail: cannot watch '/var/log/dmsg': No space left on device

To see what's using up inotify watches

for foo in /proc/*/fd/*; do readlink -f $foo; done |grep inotify |cut -d/ -f3 |xargs -I '{}' -- ps --no-headers -o '%p %U %c' -p '{}' |uniq -c |sort -nr

The first column indicates used amount and the second shows the PID of that process [sources: 1, 2].

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1  
I guess very few codes need the values higher than the default Dropbox may require a higher limit, depending on how many files you have. Iv'e raised mine with no issues. in fact, the dropbox notification (that occurs when it reaches its limit) explicitly tells you to raise it. –  Falmarri May 25 '11 at 20:55
    
Isn't there also a dnotify that works the same, but on directories? Maybe consolidate multiple files you are inotifying into a single directory? –  ultrasawblade Jun 16 '11 at 13:36
    
@ultrasawblade: am not familiar enough with the technology to give you a decent answer –  Tshepang Jun 20 '11 at 8:55
1  
@ultrasawblade- inotify replaced dnotify. dnotify was slow and buggy. inotify can be used on directories, and a directory will be "changed" when one of the files in that directory (one level deep) is modified. Directories are just files anyway. –  tjameson Jul 10 '11 at 5:14
    
Other people might run into this, too. The ps command above says error: process ID list syntax error with procps-ng version 3.3.9 on Ubuntu. –  CodeGnome Apr 19 at 18:23

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