[linux-lvm] Poor LVM dump performance

[Ross Vandegrift <ross kallisti us>]

Hi everyone,

I've run into som substantial read degredation when using dump or tar
to backup filesystems that are stored on LVM2 LVs.  Many times in the
past I have seen dump run at speeds approaching block device speeds;
50-60MiB/s, similar with tar.

I'm seeing speeds in the range of 3-4MiB/s when dumping a filesystem.
If I increase dump's blocksize to 128KiB records, speed jumps up to
about 20KiB/s, which is better but still frustratingly slow.  Going
to 1024KiB gets me about 28MiB/s.

Is this amount of performance loss expected?  My LV is just two SATA
hard disks that I put into a VG.  Stupid benchmarks with dd gives me a
raw read speed from the LV of about 60MiB/sec.

Any ideas on what's slowing down dump?


-- 
Ross Vandegrift
ross kallisti us

"The good Christian should beware of mathematicians, and all those who
make empty prophecies. The danger already exists that the mathematicians
have made a covenant with the devil to darken the spirit and to confine
man in the bonds of Hell."
	--St. Augustine, De Genesi ad Litteram, Book II, xviii, 37