On Tue, May 18, 2004 at 10:53:59AM -0400, Jonathan Dill wrote:
> If the amanda server is the only client, I'd probably turn on hardware
> compression on the tapedrive, turn off software compression, and maybe
> dump directly to tape.

Disadvantages (aside from all the other ones mentioned):
  - Hardware compression reduces the accuracy of estimates (with
    software compression, Amanda tracks the historical
    compression factor for each DLE; with hardware compression,
    Amanda has no way to know how well a given DLE compressed, so
    it -- or rather, you, when you lie to Amanda about the tape's
    length -- can only use a single guestimated compression
    factor and apply it to all the DLEs)

  - The drive might not compress data as well as gzip does; if
    that's the case, you won't get as much on each tape

  - (someone else alluded to this) Usually one doesn't care how
    long the entire backup process takes; what matters is how
    long it takes to read the data from file systems that are
    only temporarily quiescent until users start work the next
    morning, from databases that have to be shut down to get a
    consistent backup, etc.).  If your tape drive is slower than
    the dump/gtar front end of an Amanda backup, a direct-to-tape
    backup constrains the front end to the speed of the drive.
    If you use a holding disk, the front end isn't so
    constrained; Amanda can get data off the file systems at
    disk-to-disk speeds, and if the holding-disk-to-tape part of
    the process takes until mid-afternoon, it doesn't much
    matter.

As others have pointed out, if the tape drive is fast enough not
to constrain dump/gtar, it won't stream.  So tape speed is a less
important consideration than it looks at first -- whether your
tape is the bottleneck or not simply determines *which* argument
for using a holding disk applies to you :-)

--

|  | /\
|-_|/  >   Eric Siegerman, Toronto, Ont.        erics AT telepres DOT com
|  |  /
It must be said that they would have sounded better if the singer
wouldn't throw his fellow band members to the ground and toss the
drum kit around during songs.
        - Patrick Lenneau