From: lawries@btinternet.com
Date: Wed Nov 12 2003 - 14:28:07 EST
No real answer to this and I remain suprised that a disk to disk backup can possibly take longer than a tape routine but, tape technology moves on and SDLT drives are astonishingly fast.
Some common sense and good advice from Alan Rollow.
A backup is dominated the time required to do two
things:
o read the metadata and data to be backed up.
o write it somewhere.
Obvious perhaps, but it is possible to simulate each
side of this separately in your case. You can simulate
the read side by running the backup and writing to
/dev/null. You can simulate the write side by reading
from the /dev/zero device and writing to the target
file system (*).
You can time the read side of both types of backup, to
see if the options make a difference. If there's a
particular load at the time, that may also have an
affect.
(*) Simulating the tape write is harder, since the drive
probably supports compression, and /dev/zero compresses
really, really well. That would give unrealistics
write performance to the tape. Getting an infinite,
high performance data source of the right compression
ratio is hard.
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