How can I limit page cache/buffer size

By default when rsync updates your backup it creates a copy of the file and then moves it into place. To avoid this step you can have rsync write directly to your backup with the --inplace argument.

As per https://linux.die.net/man/1/rsync

–inplace

This option changes how rsync transfers a file when the file’s data needs to be updated: instead of the default method of creating a
new copy of the file and moving it into place when it is complete,
rsync instead writes the updated data directly to the destination
file.

This has several effects:

(1) in-use binaries cannot be updated
(either the
OS will prevent this from happening, or binaries that attempt to swap-in their data will misbehave or crash),

(2) the file’s data will
be in an inconsistent state during the transfer,

(3) a file’s data may
be left in an inconsistent state after the transfer if the transfer is
interrupted or if an update fails,

(4) a file that does not have write
permissions can not be updated, and

(5) the efficiency of rsync’s
delta-transfer algorithm may be reduced if some data in the
destination file is overwritten before it can be copied to a position
later in the file (one exception to this is if you combine this option
with –backup, since rsync is smart enough to use the backup file as
the basis file for the transfer).

WARNING: you should not use this
option to update files that are being
accessed by others, so be careful when choosing to use this for a copy.

This option is useful for transfer of large files with
block-based changes
or appended data, and also on systems that are disk bound, not network bound.

The option implies --partial (since an interrupted
transfer does not delete
the file), but conflicts with --partial-dir and --delay-updates. Prior to rsync 2.6.4 --inplace was also incompatible with
--compare-dest and --link-dest.