[Bug 10552] Sender checksum calculation significantly slower with compression enabled
samba-bugs at samba.org
samba-bugs at samba.org
Thu Apr 17 06:34:05 MDT 2014
https://bugzilla.samba.org/show_bug.cgi?id=10552
--- Comment #1 from John Pierman <haqthat at gmail.com> 2014-04-17 12:34:03 UTC ---
Confirmed:
Without -z avg over 3 runs (real 0m5.998s)
With -z avg over 3 runs (real 0m8.490s)
On Mon, Apr 14, 2014 at 4:10 PM, <samba-bugs at samba.org> wrote:
> https://bugzilla.samba.org/show_bug.cgi?id=10552
>
> Summary: Sender checksum calculation significantly slower with
> compression enabled
> Product: rsync
> Version: 3.1.1
> Platform: All
> OS/Version: All
> Status: NEW
> Severity: normal
> Priority: P5
> Component: core
> AssignedTo: wayned at samba.org
> ReportedBy: dougmiles at cox.net
> QAContact: rsync-qa at samba.org
>
>
> I've noticed that with the -z option enabled, comparing two files that are
> identical (or nearly so) but have different modification times takes
> significantly longer than without the -z option. It looks like the entire
> file
> is compressed as the checksum is calculated even when no data needs to be
> transmitted to the receiver.
>
> To replicate:
>
> Create test folders/files (using incompressible data to maximize effect):
> mkdir a b
> dd if=/dev/urandom of=a/a.tst bs=1M count=250
> cp a/a.tst b/
>
> run rsync without compression:
> touch a/a.tst
> time rsync -av a/ b
>
> run rsync with compression:
> touch a/a.tst
> time rsync -avz a/ b
>
> The second time with the -z option will take significantly longer, even
> though
> the source and destination files are identical apart from the modification
> times. I've also found that the latter uses much more CPU, but only on
> one of
> the two processes- the sender process I believe.
>
> Even when I added --skip-compress=tst, it made rsync much faster than -z
> alone,
> but it still took about 30% longer than omitting -z entirely.
>
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