Re: Compressed Disks

--- In [email protected], "somitcw" <somitcw@e...> wrote:
>     For reading compressed data, unless you
>  are short on CPU cycles, I believe that it
>  is faster to do less I/O.  Hercules is not
>  an exception.  Flex would not only not be
>  an exception, but any MIPS limits imposed
>  by a zOS license might leave extra Intel
>  CPU cycles for other work?

When the ratio of processor speed vs disk speed is great enough,
as they are now on the average personal computer, then real
differences can be observed when using compressed disk images
vs normal disk images.  If the ration drops by a magnitude or
two then then differences might not be so noticable.

Note that one of the compression algorithms used by the code
is `none'.  Even without compressing track images a compressed
disk image can be smaller than a normal disk image because only
the space required for the track is used.  In regular images
each track has the maximum possible size reserved for it, although
it is very unlikely that any track will actually be the maximum
size.

When we read from a compressed disk image we only read the
number of bytes occupied by the track; when we read from a normal
disk image we read the maximum size.  (When we write to a normal
disk image, we know the low/high updated offsets in the track so
we don't actually write the maximim size).

Normally, though, tracks are compressed using zlib or bzip2.  A ROT
is that a compressed disk image occupies 20% the space occupied by
a normal image.  This also means that the amount of data read 
for a compressed disk is about 1/5 the amount for a normal disk.
Writes can get bogged down since compression is a lot more cpu-
intensive than decompression.  In the next release of hercules,
the compression thread runs at a lower priority than the cpu thread,
and can detect intances of contention with the cpu thread (stress)
and relax the compression parameters (less cpu/more space).

Finally, compressed images fit more easily onto removable media.

Greg


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