A dump performance hint: the block size really matters

October 23, 2006

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).

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 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.)

Written on 23 October 2006.
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Last modified: Mon Oct 23 17:26:47 2006
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