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I am currently copying a large number of directories and files recursively on the same disk using cp -r.

Is there a way to do this more quickly? Would compressing the files first be better, or maybe using rsync?

  • If this is on zfs, you can make a snapshot, which is practically instantaneous. The cost of the copy (both in time and in disk space) is then only paid when one of the sides is modified. I don't know what commands to use for this, I encourage someone who does to post an answer explaining how to do it. – Gilles 'SO- stop being evil' Aug 25 '16 at 22:08
  • If you could post the output of iostat while this copy operation is running, you might get more help from readers. Assuming you're running on Solaris from the /solaris tag, post several lines from iostat -sndzx 2. That will emit an output line every 2 seconds, with the first line being not very useful. Again, that needs to be run while your cp -r ... command is running. – Andrew Henle Aug 27 '16 at 11:30
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I was recently puzzled by the sometimes slow speed of cp. Specifically, how come df = pandas.read_hdf('file1', 'df') (700ms for a 1.2GB file) followed by df.to_hdf('file2') (530ms) could be so much faster than cp file1 file2 (8s)?

Digging into this:

  • cat file1 > file2 isn't any better (8.1s).
  • dd bs=1500000000 if=file1 of=file2 neither (8.3s).
  • rsync file1 file2 is worse (11.4s), because file2 existed already so it tries to do its rolling checksum and block update magic.

Oh, wait a second! How about unlinking (deleting) file2 first if it exists?

Now we are talking:

  • rm -f file2: 0.2s (to add to any figure below).
  • cp file1 file2: 1.0s.
  • cat file1 > file2: 1.0s.
  • dd bs=1500000000 if=file1 of=file2: 1.2s.
  • rsync file1 file2: 4s.

So there you have it. Make sure the target files don't exist (or truncate them, which is presumably what pandas.to_hdf() does).

Edit: this was without emptying the cache before any of the commands, but as noted in the comments, doing so just consistently adds ~3.8s to all numbers above.

Also noteworthy: this was tried on various Linux versions (Centos w. 2.6.18-408.el5 kernel, and Ubuntu w. 3.13.0-77-generic kernel), and ext4 as well as ext3. Interestingly, on a MacBook with Darwin 10.12.6, there is no difference and both versions (with or without existing file at the destination) are fast.

  • Did you account for the source file contents potentially being held in cache? – Andrew Henle Jul 8 '18 at 18:17
  • @AndrewHenle: good point, but same conclusions when clearing the cache (using sudo sh -c 'sync && echo 3 > /proc/sys/vm/drop_caches') before every command. Just adding ~3.8s to all the numbers above. The delta between cp to an existing file vs to an non-existent destination is as above: ~7s. – Pierre D Jul 9 '18 at 4:02
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On the same partition (and filesystem) you can use -l to achieve hard links instead of copies. Hard link creation is much faster than copying things (but, of course, does not work across different disk partitions).

As a small example:

$ time cp -r mydir mydira

real    0m1.999s
user    0m0.000s
sys     0m0.490s

$ time cp -rl mydir mydirb

real    0m0.072s
user    0m0.000s
sys     0m0.007s

That's a 28 times improvement. But that test used only ~300 (rather small) files. A couple of bigger files should perform faster, a lot of smaller files slower.

  • It's on the same partition – CJ7 Aug 25 '16 at 2:23
  • What are hard-links? I need actual copies of the files to play around with. – CJ7 Aug 25 '16 at 2:24
  • Hard links make each filename map to the same file; they're not copies. If you modify the new name you modify the original. – Stephen Harris Aug 25 '16 at 2:27
  • @CJ7 - Hard links are just extra inodes pointing to the same data. If you change the copy the original file is changed too. – grochmal Aug 25 '16 at 2:27
  • Also I'm on solaris and my cp does not have a -l option. – CJ7 Aug 25 '16 at 2:28
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Copying a file on the local disk is 99% spent in reading and writing to the disk. If you try to compress data then you increase CPU load but don't reduce the read/write data... it will actually slow down your copy.

rsync will help if you already have a copy of the data and bring it "up to date".

But if you want to create a brand new copy of a tree then you can't really do much better than your cp command.

  • They could use CoW snapshots. That's essentially creating a new copy of the files and you only have the initial snapshot operation and subsequent increased latency for writes to the new "copies" – Bratchley Aug 25 '16 at 2:32
  • @Bratchley did you read the question? CoW does not meet the requirements (and it's Solaris, anyway). – Stephen Harris Aug 25 '16 at 2:37
  • Well, you might do some form of tmpfs, mount it somewhere, CoW onto it, and then edit the "copies". But (1) that's horribly far fetched, and (2) not on solaris. – grochmal Aug 25 '16 at 2:39
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    @grochmal If it's reasonably current version of Solaris, it's almost certainly going to be using ZFS which supports snapshotting. There are then a variety of ways to get those "copies" to show up in a desired part of the filesystem. – Bratchley Aug 25 '16 at 2:42
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    ha yeah. Solaris is the birthplace for ZFS ;-) – Bratchley Aug 25 '16 at 2:45

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