by >When backing up to a DDS tape which is large enough to contain all of a
>site's files as is, uncompressed, is it measurably slower (or faster) to
>let the DDS drive exercise its internal compression routine?
>(Obviously I could test this myself, but I'm trying not to reinvent any
>wheels, and am interested in any explanations of why the answer is
>whatever it is.)
mostly off topic :)
Tape drives have a fixed transfer rate to the tape. Say 5mb/sec.
If you are getting 2:1 compression, every 5mb to tape is 10mb of
your data.
Data is streaming through the controller at 10mb/sec and compressed
data is going onto the tape at it's steady old rate of 5mb/sec.
So, your backup runs twice as fast with 2:1 compression, subject to
other limiting factors like file reads and controller and bus
performance.
With a pci bus and decent controller they are not the bottleneck.
It will be a combination of read speed and compression ratios.
- Binary data (executables and the like) seem to compress about 2:1.
- Database and text data often compress much more.
- Compressed data doesn't compress further (.Z, .gz, many graphic
formats).
- YMMV
I've never noticed uncompressable data to a tape drive in compressing
mode to run any slower than in non-compressing mode.
So, there is no reason beyond compatability ( with older non-compressing
or proprietary compression schemes ( which are no longer used as far
as I know)) to use non-compressing mode. IMHO.
--
Do two rights make | Kevin Smith, ShadeTree Software, Philadelphia, PA, USA
a libertarian | 001-215-487-3811 shady.com,kevin bbs.cpcn.com,sysop
| dvtug.org,kevins--Delaware Valley Transit Users Group