Re: How can we check that a compressed file is rsyncable ?
On 2019-06-25, rhkramer@gmail.com <rhkramer@gmail.com> wrote:
> On Mon, Jun 24, 2019 at 12:42:37PM -0000, Curt wrote:
>> On 2019-06-22, Andy Smith <andy@strugglers.net> wrote:
>> > I am not aware of any other compression tool that offers to do what
>> > gzip's --rsyncable option does, but I owuld be interested if there
>> > are some that I overlooked.
>>
>> zstd introduced an '--rsyncable' flag with version 1.3.8 (available from
>> stretch-backports for those using stable).
>
> I guess I'd like to know what makes a compressed file rsyncable -- I guess that
> means it is compressed so that a single change doesn't change many or all
> parts of the file.
>
> If an encryption method attempted to do the same thing (to create an rsyncable
> encrypted file), that might compromise the encryption of the file.
There's something called rsyncrypto of which you may not have heard (in
a Debian repository near you).
The home page, however, is MIA (which may or may not be weird or disquieting).
http://rsyncrypto.lingnu.com
I get (after a browser warning because there's no s after the p):
No site configured at this address
>From the man:
rsyncrypto is a utility that encrypts a file (or a directory structure) in
a way that ensures that local changes to the plain text file will result in
local changes to the cipher text file. This, in turn, ensures that doing
rsync to synchronize the encrypted files to another machine will have only a
small impact on rsync's wire efficiency.
> (And, I think it has been at least implied in the thread, that any file can be
> transferred by rsync -- to be rsyncable I assume (I know) means that there are
> enough unchanged "segments" of the file after a change to the unencrypted file
> that the rsync algorithm is worthwhile (the computational cost of the rsync
> algorithm is paid back in terms of a shorter overall transfer time for the
> file).
>
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