Everhart,Glenn
From:	Everhart,Glenn
Sent:	Thursday, March 12, 1998 9:31 AM
To:	Info-VAX@Mvb.Saic.Com
Subject:	Re: File/Disk compression for OpenVMS?
In adition to things like zip and the compressing disks mentioned (one
on the freeware
CD for example that makes a compressed, readonly image of a disk that
acts like
the original disk, and will work over networks), Montagar Software sells
a
package that compresses and decompresses files. It uses some RMS hooks
in a clever and original way (way to go, Dave Cathey) to compress files
and present
them decompressed on the fly. Directories and so forth are not altered
from normal,
but files are stored compressed, with the necessary semantic tag ACE to
get the
package to always show them decompressed.

Check out ftp.montagar.com or I suspect www.montagar.com for this. Dave
also
keeps copies of the sigtapes around. My compressing readonly disk image
(which
works fine for CD images and so on too) is in file cmphighc.zip or
something similar
on the sigtapes. Mine comes with sources & is free. 

The problems a r/w compressing disk faces are that it must manage the
compressed
storage somehow. This requires a meta-filesystem underneath. Even if you
play a lot
with log oriented filesystem schemes, the meta filesystem must
nevertheless manage
storage placement & allocation and retrieval and adds overhead.
Sometimes you may
not notice this for bursty writes, if an underlying cache of some kind
masks the
added I/O, but total I/O operation counts will see this.

Dave's solution avoids this second level storage problem and thus seems
likely to be
able to work much more smoothly.

If on the other hand you want to handle archival information compactly,
the freebie I
wrote is incredibly handy. I use it to keep some virtual CDs online and
to squeeze more
info onto CDs I write onto CD-R backups. Using it I can often get CD-Rs
that contain what
amounts to 1.5 to 2 gigabytes of stuff, all in the container files, and
all able to be read
as if it were on a local disk if I want. The format can be read on Vax
or Alpha VMS (and
wouldn't be hard to decode on something else; it just has a file of
index info giving
the start LBN and size of data chunks on the data container file, the
data being
comressed in 32 block chunks (block=512 bytes).) (There's also a
compression flag;
data that won't shrink by a block or more is stored as is so it won't
grow.)

Glenn Everhart