Paul Slootman <paul+rs...@wurtel.net> wrote:
> On Wed 18 Dec 2013, Kevin Korb wrote:
> > Also, rsync's -c is rather dumb as it computes checksums for files
> > that have different sizes so they can't possibly be the same and it
> > computes checksums for files that only exist on one end and therefore
> > has nothing to compare them to.
>
> The list of files on the source is generated and transferred to the
> destination before rsync knows that the destination file is different.
>
> To make rsync checksum only the files with same size would mean changing
> the filesystem scan to a two-pass thing (send the list of filenames plus
> their sizes, wait for the destination to tell you what files need
> checksumming, do that and send the filenames again, now with checksum
> data), and retransferring file metadata again.
That seems to imply that avoiding unnecessary checksum calculations
would double the protocol overhead, which I think is overly pessimistic.
IIUC, the current protocol, without checksums, amounts to (greatly
simplified):
Sender Receiver
I have (name1,meta1)
(name2,meta2), ...
I need #s 3,8,13, ...
Send requested files
Write files to destination as received
To do checksums only when needed:
Sender Receiver
Checksums have
been requested.
I have (name1,meta1)
(name2,meta2), ...
Based on sizes, I know
I need #s 3,13, ...
and I may need (#5,checksum5),
(#8,checksum8), ...
Send known-needed
For each "may need"
compute checksumN
if match
send "#N matches"
else
send "#N no match"
send #N data
Write files to destination as received
If that is "a two-pass thing", the current protocol must involve
1.9 passes :)
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