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How does one find large files that have been deleted but are still open in an application? How can one remove such a file, even though a process has it open?

The situation is that we are running a process that is filling up a log file at a terrific rate. I know the reason, and I can fix it. Until then, I would like to rm or empty the log file without shutting down the process. Simply doing rm output.log removes only references to the file, but it continues to occupy space on disk until the process is terminated. Worse, after rming I now have no way to find where the file is or how big it is! Is there any way to find the file, and possibly empty it, even though it is still open in another process?

I specifically refer to Linux-based operating systems such as Debian or RHEL. Thanks.

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If you know the pid then you can use lsof -p <pid> to list its open files and their sizes. The deleted file will have a (deleted) next to it. The deleted file will be linked at /proc/<pid>/fd/1 probably. I don't know how to make a process stop writing to its file descriptor without terminating it. I would think that would depend on the process. –  donothingsuccessfully Mar 20 '13 at 8:15
Thanks. How might one get the PIDs of all rmed files that are still open? –  dotancohen Mar 20 '13 at 8:17
@donothingsuccessfully The "deleted" tag reported by lsof is Solaris specific, in fact Solaris 10 or later only. The OP did not specify what operating system he is using. @dotancohen On Solaris you can pipe the output of lsof to search for deleted, eg lsof | grep "(deleted)". When there are no more processes holding a deleted file open, the kernel will free up the inode and disk blocks. Processes do not have "handlers" by which they can be notified that an open, essentially locked file, have been removed from disk. –  Johan Mar 20 '13 at 8:43
Thank you, I added a Linux tag and mentioned Linux in the body. –  dotancohen Mar 20 '13 at 9:11
@Johan, the lsof | grep '(deleted)' works on Linux as well. On Linux, you can be notified of file deletion (even files that already don't have any entry in any directory other than /proc/some-pid/fd anymore) with the inotify mechanism (IN_DELETE_SELF event) –  Stephane Chazelas Mar 20 '13 at 11:14
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2 Answers

up vote 12 down vote accepted

If you can't kill your application, you can truncate instead of deleting the log file to reclaim the space. If the file was not open in append mode (with O_APPEND), then the file will appear as big as before the next time the application writes to it (though with the leading part sparse and looking as if it contained NUL bytes), but the space will have been reclaimed.

To truncate it:

: > /path/to/the/file.log

If it was already deleted, on Linux, you can still truncate it by doing:

: > "/proc/$pid/fd/$fd"

Where $pid is the process id of the process that has the file opened, and $fd one file descriptor it has it opened under (which you can check with lsof -p "$pid".

If you don't know the pid, and are looking for deleted files, you can do:

lsof -nP | grep '(deleted)'

Or (on Linux):

find /proc/*/fd -ls | grep  '(deleted)'

Or to find the large ones with zsh:

ls -ld /proc/*/fd/*(-.LM+1) | grep '(deleted)'

An alternative, if the application is dynamically linked is to attach a debugger to it and make it call close(fd) followed by a new open("the-file", ....).

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Thank you Stephane. However this of course depends on knowing the PID which I don't and won't know. I'll see where I can take it, though. –  dotancohen Mar 20 '13 at 9:13
There's also a truncate command that does the same thing more explicitly. –  Tobu Mar 20 '13 at 9:15
@dotancohen Stephane edited to include info on how to do this when the pid is not known. –  David Kohen Mar 20 '13 at 10:48
very nice answer! +1 for it. However, it won't work on all unix systems (for ex: AIX 6.1 + lsof 4.82, at least, where lsof doesn't show the path to the file, and /proc/pid/fd/x acts apparently differently (not sure of the details yet)) (I know it is tagged linux only, but I'd welcome a "most unix" answer very much!) –  Olivier Dulac Mar 20 '13 at 13:07
@StephaneChazelas: thanks. I found a way to list all PIDs which have a file open on each partitions : df -k | awk 'NR>1 { print $NF }' | xargs fuser -Vud (and then easy to send signals to the offenders to force them to release the fd) –  Olivier Dulac Mar 20 '13 at 18:56
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It's up to the file system driver to actually free the allocated space, and that will usually happen only once all file descriptors referring to that file are released. So you can't really reclaim the space, unless you make the application close the file. Which means either terminating it or playing with it "a bit" in a debugger (e.g. closing the file and making sure it is not opened/written to again, or opening /dev/null instead). Or you could hack the kernel, but I would advise against that.

Truncating the file as Stephane suggests might help, but the real outcome will also depend on your file system (for example pre-allocated blocks will likely be freed only after you close the file in any case).

The rationale behind this behaviour is that the kernel wouldn't know what to do with data requests (both read and write, but reading is actually more critical) targeting such a file.

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As Linux supports sparse files on most file systems, the behaviour is well-defined and the disk driver can really free disk space. I have tested it for ext3 and ext4, and it works like Stephane wrote. –  jofel Mar 20 '13 at 10:14
What makes you say that truncating a file will not reclaim preallocated blocks? Truncating is meant to deallocate data, I don't thing there's any ambiguity with that. –  Stephane Chazelas Mar 20 '13 at 11:22
The file system may keep the blocks allocated to save time later (especially if the file still remains open), especially when it was big enough before truncating. At least that's what XFS seems to be doing. –  peterph Mar 20 '13 at 12:53
Thank you Peter. I am glad that you address the "why" in this post. –  dotancohen Mar 21 '13 at 6:09
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