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

[Doug Robinson]

Wayne:

Thank you for responding.

On Thu, Mar 13, 2014 at 8:37 PM, Wayne Davison <wayned at samba.org> wrote:

> On Tue, Mar 11, 2014 at 3:11 PM, Doug Robinson <doug.robinson at wandisco.com
> > wrote:
>
>> 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?
>
>
> See patches (in order of recommendation): db.diff, checksum-updating.diff,
> checksum-xattr.diff.
>

Will do.


> I personally use db.diff in one situation at work combined with a sqlite
> DB on the source and destination machines.  You just need to periodically
> weed out any old inode values (via rsyncdb --clean /dirs) if things start
> to bloat.  In the future I'd like to see the db.diff code included by
> default as loadable libraries, which would allow someone to install plain
> rsync and only also install sqlite-using rsync and/or mysql-using modules
> if they want the extra functionality.  There is also a plan to eventually
> have the db code map the inodes in the db to paths for things like rename
> optimizations.
>

Nice.


> That said, all these patches currently do is cache checksums.  The db
> patch's default strict checking only uses a cached inode's info if the
> size+mtime+ctime all match what we knew about the file when it was cached
> (which makes it pretty safe).
>

Seems fine unless xattrs are in play.  I saw your comment in a prior
posting abut ctime - and it makes a lot of sense.  Thank you.


>  If you switch to a more lax algorithm (no ctime) you need to be extra
> sure the files don't get updated in some way as to leave the file matching
> the laxer inode info (e.g. only let rsync make changes to the files and/or
> make sure that modify timestamps always increase so that there is no chance
> of accidentally matching an older inode record).
>

We're not dealing with xattr so I plan on using the size+mtime+ctime match.


> If you're wondering how an mtime-using algorithm helps your use case, keep
> in mind that the mtimes don't need to match between hosts, just between
> each host's files and its db cache (and any non-matching or missing ones
> get (re)computed to the new checksum).
>

That was the way that I understood it - glad to read I'm on the right path.


> I'll also point out that if you want to use sqlite, I recommend you use
> the very latest db.diff (from the git patches repo) since it has a change
> that alleviates locking contention between the multiple rsync processes in
> a single copy (you can't really share the db between simultaneous rsync
> copies due to sqlite's poor multi-process locking -- use mysql for that).
>

I'll have to consider my use case(s) to determine the right of sqlite vs.
mysql.  Thank you for the head's up.


> The rsyncdb manpage has info on initializing the db, noting mounts,
> maintenance, etc.
>
> The other patches might also be useful to you, so feel free to check them
> out:  https://git.samba.org/?p=rsync-patches.git
>

Excellent.  Thank you.

Doug
-- 
Doug Robinson

WANdisco // *Non-Stop Data*

t. 925-396-1125
e. doug.robinson at wandisco.com

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