Caching {filePath,mtime64,checksum} values to speed up execution-time

[Kevin Korb]

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- --checksum should not be used during normal rsync operations.  It is
for special cases only.

Rsync can still have a lot of overhead getting the timestamps via
stat() but that can't really be helped.

I don't really understand how file mtimes would be cached.  How would
rsync know what mtimes don't match the cache without checking
stat()ing the files and then the job is already done so the cache
wouldn't accomplish anything.

On 03/11/2014 06:11 PM, Doug Robinson wrote:
> Folks:
> 
> When using rsync to copy huge amounts of data I've found that a 
> significant amount of time is spent computing the checksums.
> Sometimes hours, ... sometimes days - it depends on the total
> amount of data checked!  And after that sometimes it's only a few
> files that need to be updated.
> 
> I've pulled the latest git (rsync-3.1.1pre1) and didn't see
> anything to address this (or I missed it?).
> 
> I was wondering what folks thought of a proposal to enhance rsync
> to be able to create and maintain a cache of {filePath, 64-bit
> mtime, checksum} beforehand on both source and target systems and
> then use that cache later on when asked to sync the two systems
> together?  Then cache entry validation would be a quick stat64() to
> make sure that the 64-bit mtime didn't change before sending the
> checksum over the wire for comparison.
> 
> Clearly the cache would need to be completely invalidated (or 
> re-created) if the file system became corrupt.  That could be
> handled via an "rm -rf" of the cache.
> 
> Thoughts?
> 
> Thank you.
> 
> Doug -- WANdisco // /Non-Stop Data/
> 
> t. 925-396-1125 e. doug.robinson at wandisco.com
> <mailto:doug.robinson at wandisco.com>
> 
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