Proposal for solving CD Size problems

[Phillip Lougher]

On 9/28/06, Phillip Lougher <phillip.lougher at gmail.com> wrote:

> 1. Block to be decompressed is on disk only.  Overhead to get
> decompressed data is seek-time + block I/O + decompression, and this
> can't be done in parallel.  Five times slower decompression even in
> this case is no loss only if the ratio of seek+I/O time to
> decompression time makes decompression overhead negligible, i.e 5
> times nothing = nothing.  Taking some measurements I did a couple of
> years ago (http://tree.celinuxforum.org/CelfPubWiki/SquashFsComparisons),
> for example reading a  squashfs re-encoded Ubuntu livecd from CDROM
> took 5 minutes 15.46 seconds with System time of 51.12 seconds, i.e.
> 16% of the time was decompression overhead.  Five times this is
> certainly no loss, figuring this into the stats would make something
> like total overhead 8 minutes 39 seconds with decompression time of 4
> minutes 15 seconds, or 2 times slower.  Reading from hard disk where
> seek-time and block I/O is a smaller percentage of overhead, makes the
> performance loss even worse.

8 minutes 39 seconds as opposed to 5 minutes 15.46 seconds == 1.65
times slower overall.  This is sequential access, real-life random
access will be worse as data will need to be re-decompressed from the
block cache, hitting 5 times slower performance (item 2).

Phillip