rsync to servers highly sensitive to IO load

[Wayne Davison]

On Fri, Jul 11, 2008 at 10:10:16AM -0400, Jeff Woods wrote:
>     --bwlimit=17 --partial --append

You only want to use --append if you can guarantee that the files will
not have any changes in the existing data on the receiving side.  A
modern rsync does not compute a full-file checksum for --append unless
you use a second --append, since that slows down the appending.

If you're using -c (--checksum), you probably don't want to do that,
since that's super slow.  Just use -t (--times) and let the normal
size+mtime check look over things for you.  If you find that you really
need checksumming, you might want to look at the db.diff in the patches
dir that lets you cache checksums in a DB (i.e. SQLite or MySQL) and
associate them with unchanged files (since it matches a file's size,
mtime, ctime, and inode, it is safe).

If the source of the I/O is rsync's scanning of the directories (not the
checksumming of the files), you may want to look into the slow-down.diff
file in the patches dir, as that provides a way to get rsync to do its
directory scanning more slowly.

> negating the need to re-read the entire file for a post-transfer
> checksum

There is no such thing in rsync, since it computes the checksum as the
file is written.

..wayne..