[Beowulf] Rsync - checksums

[Stu Midgley]

Are you rsyncing over ssh?  If so, get HPN-SSH and use the non-cipher.
MUCH faster again :)

On Tue, Jun 18, 2019 at 11:00 PM Bill Wichser <bill at princeton.edu> wrote:

> Well thanks for THAT pointer!  Using --checksum-choice=none results in
> speedup of somewhere between 2-3 times.  That's my validation of the
> checksum theory things have been pointing towards.  Now to get xxhash
> into rsync and I think we are all set.
>
> Thanks,
> Bill
>
> On 6/18/19 9:57 AM, Ellis H. Wilson III wrote:
> > On 6/18/19 9:16 AM, Bill Wichser wrote:
> >> Stock RH 7 version, rsync-3.1.2-6.el7_6.1.x86_64.  We've tried a
> >> number of recompiles.  gcc, Intel.  The only thing between identical
> >> compiles was the md4 vs md5.
> >>
> >> /bin/rsync -lptgoDAH -v --numeric-ids -d --relative --delete
> >> --delete-after --files-from=...
> >>
> >> I'm not asking for help.  Just if anyone had attempted to change the
> >> algorithm into something much faster.
> >>
> >> I refer you to this project https://cyan4973.github.io/xxHash/ where
> >> there is a table of speeds.  Regardless of what anyone might
> >> speculate, we are pursuing this route of changing out the algorithm.
> >> Maybe it's all for naught.  Maybe it isn't.  But in a few weeks
> >> hopefully we'll have determined.
> >
> > Very interesting.  From the rsync man page:
> >
> > "Note that rsync always verifies that each transferred file was
> > correctly reconstructed  on  the  receiving  side  by checking  a
> > whole-file checksum that is generated as the file is transferred, but
> > that automatic after-the-transfer verification has nothing to do with
> > this option’s before-the-transfer "Does this file need to be updated?"
> > check."
> >
> > So it sounds like you have sufficient churn in large files that the
> > checksum validation post-transfer is your bottleneck.  Short of hacking
> > rsync to use a faster algorithm, your remaining choice is to use the
> > --checksum-choice=STR and set it to none, and then perform your own
> > hashing out-of-band to check the transferred data using the list you
> > have provided via in files-from.  This will nerf rsync's ability to do
> > delta-transfer, which may be ok depending on the nature of your churning
> > files.  If your pipes are huge (atypical for DR), your CPU is weak, and
> > your churning data is mostly completely new or completely changed files,
> > --checksum-choice=none may work very well for you.
> >
> > Best,
> >
> > ellis
> >
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-- 
Dr Stuart Midgley
sdm900 at gmail.com
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