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I'm getting a diff: memory exhausted error when trying to diff two 27 GB files that are largely similar on a Linux box with CentOS 5 and 4 GB of RAM. This is a known problem, it seems.

I would expect there to be an alternative for such an essential utility, but I can't find one. I imagine the solution would have to use temporary files rather than memory to store the information it needs.

  • I tried to use rdiff and xdelta, but they are better for showing the changes between two files, like a patch, and are not that useful for inspecting the differences between two files.
  • Tried VBinDiff, but it is a visual tool which is better for comparing binary files. I need something that can pipe the differences to STDOUT like regular diff.
  • There are a lot of other utilities such as vimdiff that only work with smaller files.
  • I've also read about Solaris bdiff but I could not find a port for Linux.

Any ideas besides splitting the file into smaller pieces? I have 40 of these files so trying to avoid the work of breaking them up.

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what version of xdelta did you try? xdelta3 or xdelta1? – nmuntz Aug 10 '10 at 16:17
It was version 1.1.4. Does xdelta3 provide different functionality? I just checked the online doc and it still seems to be about providing "deltas". – Tom B Aug 11 '10 at 15:35
See also this answer: – unhammer Mar 5 '14 at 9:19

I found this link

diff -H might help, or you can try installing the textproc/2bsd-diff port which apparently doesn't try to load the files into RAM, so it can work on large files more easily.

I'm not sure if you tried those two options or if they might work for you. Good luck.

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Does this help for anybody out there? For me, same failure... – rogerdpack Jul 8 '14 at 17:54
For anyone wondering: diff -H is an undocumented and deprecated alias for diff --speed-large-files. – a3nm Feb 27 at 22:47

cmp does things byte-by-byte, so it probably won't run out of memory (just tested it on two 7 GB files) -- but you might be looking for more detail than a list of "files X and Y differ at byte x, line y". If the similarities of your files are offset (e.g., file Y has an identical block of text, but not at the same location), you can pass offsets to cmp; you could probably turn it into a resynchronizing compare with a small script.

Aside: In case anyone else lands here when looking for a way to confirm that two directory structures (containing very large files) are identical: diff --recursive --brief (or diff -r -q for short, or maybe even diff -rq) will work and not run out of memory.

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nice, I think -q is the key here, somehow not having it can require diff to put the whole file (or at least whole lines) into memory... – rogerdpack Jul 8 '14 at 17:55
This solved my issue. Thank you. – dimitko Dec 24 '15 at 14:06

If the files are identical (same length) except for a few byte values, you can use a script like following (w is the number of bytes per line to hexdump, adjust to your display width):

while read -ru7 x && read -ru8 y;
  [ ".$x" = ".$y" ] || echo "$x | $y";
done 7< <(od -vw$w -tx1z FILE1) 8< <(od -vw$w -tx1z FILE2) > DIFF-FILE1-FILE2 &


It's not very fast, but does the job.

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