4

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?

|improve this question
  • 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
2

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.

|improve this answer
  • 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
1

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.

|improve this answer
  • 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
1

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.

|improve this answer
  • 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
  • 1
    @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
  • 1
    ha yeah. Solaris is the birthplace for ZFS ;-) – Bratchley Aug 25 '16 at 2:45

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.