Matt, if you think you're actually getting 4:1 real world compression out of
a modem, I suggest you read some of the research on the subject. Compression
is highly variable, depends on the data, and 4:1 is rarely achieved. What's
more it induces considerable latency, often exceeding the value of the
compression. The irony is that lower compression rates take less processing
time and power than attempting to achiieve higher compression.
Now latency isn't as big an issue for backup as it is for a highly dynamic,
realtime application like modems support. However, the data pipe is vastly
greater. Fast ethernet has a thousand times the bandwidth of a 56 k modem,
about twice the uncompressed streaming rate of a 7000 series DLT drive.
What does this all mean? Basically if you have a fast ethernet connection
with no routers inbetween the host and station the hardware should support
uncompressed streaming to tape. Compressed files will probably not quite
stream, but to do software compression at the clients would require very
fast clients; it's one of the many specialized tasks that is easier to do in
hardware - at the tape drive. You're most likely constrained by disk access
speed anyway.
BTW, this is an old question, hardly unique to Dantz. Client compression has
been tried, and can be a useful tool under some circumstances, but it
usually causes as many problems as it solves. And you're right about
aquiring vs creating compression technologies. It usually doesn't make sense
to reinvent that particular wheel. Other methods of improving data feeds to
the tape drive include intelligent backup (something Retrospect is very good
at) and multiplexing data sources. This last option has it's proponents, but
tends to be rather cranky in practice - and restores are very slow.
-----Original Message-----
From: matt barkdull
To: retro-talk
Sent: 2/26/01 10:52 AM
Subject: RE: Transfer Rates - Dantz Help?
A month or so ago I wrote a rant about how Dantz should work with
Alladin and come up with a better compression scheme.
I know that the on-fly compression is difficult to maintain speed,
but it seems like better than 2:1 should be possible. I'm not much
of a wiz at all with compression, however I see modems getting
v.42bis (4:1) on the fly, it seems like a little work and this should
be possible for client and server as well.
Yes, I know that advertised and what you really get are totally
different, but all I know is that if something is advertised at 4:1,
it will be more likely to get at least 2:1 that 2:1 is likely to.
Alladin is cross platform. Dantz covers the same platforms.
Yes, most people use hardware compression, but this is mostly because
the hardware and software compression are likely to get the same
results.
Why would anyone want to write their own compression? I mean, a
license deal from Alladin, who's been doing it since the early days
of Mac, would seem like it would be far more cost effective.
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