On Thu, Dec 15, 2016 at 12:11 PM, Rahul Amaram
<amaramra...@users.sourceforge.net> wrote:
>
> 1. What I am trying to achieve: transfer lesser traffic for full-backups. My
> understanding is that --checksum transfers the file content only if there
> are changes unlike --ignore-times but there are certain tradedoffs (Ref:
> http://serverfault.com/questions/211005/rsync-difference-between-checksum-and-ignore-times-options)
Rsync is only going to transfer the differences in the file content -
and if there are no changes it won't transfer any either way. That's
kind of the point of using rsync. The difference is just in how
completely it checks each file. For incrementals, without the
--ignore-times, it quickly skips files where the directory entries
have the same timestamp and length as the current copy. With
--ignore-times, all files are read on the target system and compared
to the backup with rsync's block-checksum algorithm - but only changed
blocks would be copied over. If you have activated checksum caching,
after the 2nd full run, the server side will not have to read and
uncompress its copy of the file to compute the checksums again, but
the files are always read on the target system which takes some time
but not much bandwidth.
> 2. In BackupPC 4.0.0 alpha3, the option for full-backup is changed from
> --ignore-times to --checksum. If as you said "--checksum" option just slows
> down the transfer and makes things less accruate , I really wouldn't know
> why the author would make --checksum the default option.
In the 3.x versions, the server side is using a perl implementation of
rsync in order to read the compressed stored backup while chatting
with the remote native version reading the uncompressed original.
It does not include the --checksum option.
> What I was expecting from this mailing list, was some help on how I could
> use the --checksum option. Just the simple message "backuppc 3.3.1 uses
> File::RsyncP which does not support --checksum" would have been much more
> appreciated rather than criticizing my mail so strongly. I have been using
> backuppc for 8 years, and it is what I recommend at the organizations that I
> work for, but this kind of hostility would discourage people from adopting
> it.
Holger likes the details to be right. Often that turns out to be important...
--
Les Mikesell
lesmikes...@gmail.com
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