== A _dump_ performance hint: the block size really matters

I have always more or less ignored dump's _-b_ argument, assuming it was
something only relevant for output to tapes (which I never do; even our
backups that eventually wind up on tapes get sent to standard output for
mangling by [[Amanda http://www.amanda.org/]]).

Except today I was getting disappointing dump speeds from a system with
fast disks, so I decided to experiment with it. And boy have I been
wrong.

| ~~_dump_ block size~~	| ~~megabytes/sec~~
| 10K (default)		| 6 Mb/s
| 256K			| 32 Mb/s
| 512K			| 37 Mb/s
| 1024K			| 52 Mb/s

(This is on a [[disk system ModernDiskPerformance]] that hits 69 MB/s on
streaming reads.)

I got these numbers from a filesystem with a fair number of large files,
but my home directory partition (complete with my MH folders) still did
40 Mb/s at 1024K. Since I am soon to dump and restore this machine to
another one (don't ask) and I have a gigabit network link that I could
use for this with some work, this is a quite timely discovery.

(Despite all the bad things that people say about it, the _dump_ family
remains my favorite tool for backing up and copying entire filesystems.
I feel that dumping from outside the filesystem, with full access to the
exact on-disk representation of the files you're working on, is the most
reliable way in general to get an exact copy of everything.)