#79 7zip compression options confusing

[Anonymous]

The 7z format offers lots of compression options,
which is good; but many of the options are not very
well explained, which is bad. For example, when
creating 7z archives, online help mentions the use
of streams, but only says some algorithm support
multiple streams, but which one is it? Later it was
shown as BCJ2, but what about other methods?
Memory requirements for indivual methods are
explained, but when BCJ2 is used, it was a little bit
unclear. Does compressing all 4 streams using the
same method needs 4 times the memory, or just
the maximum amount of memory used by one
stream, or anywhere in between? Speaking of
compression requirement, what about
decompression and crypto? Unless there is an
interactive tool to help users to calculate memory
requirements, sooner or later people may run found
the archives become useless because of
oversighted compression options. Speaking of
memory requirements, how much memory does 7-
Zip use for BZip2 compression? It's not on the
manual.

Manual mentioned BCJ, but not on the website. I
assume it is obsolete, but if that is the case, it is
better to say it so than just hiding it. Anyway, I
can't find out if 7-Zip can still extract 7z archives
with BCJ method from the manual.

There is a problem of creating BZip2 archives with
7-Zip. 7-Zip just says 'invalid option!' Not many
people realize bzip2 can't store more than one file,
and nowhere in the manual mentions such
limitations, and the error message surely doesn't
help.

But what adds to the confusion was lack of profiles
support. Given 7-Zip offers lots of compression
options, the ability to store profile settings is
extremely important. But what 7-Zip file manager
has is very insufficient. It only stores one line of
switches, and it is shared when other compression
methods and archive formats are chosen! Unless
someone uses only one archive format with only
one compression method (which I highly doubt
anyone will do with 7-Zip), exploiting 7-Zip's
capabilities is an exercise of frustration and
hassles.

Anyway, keep up the good work.

[Igor Pavlov]

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Each stream needs memory.
Crypto is fast and it uses small part of memory.
bzip2 - 10 MB.
7z can create & extract BCJ.
Options in 7-zip create archive dialog box are not
shared.

[Nobody/Anonymous]

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> bzip2 - 10 MB.

bzip2 manual, stated memory used for compression is 400k
+ ( 8 x block size ), so 10MB will create incompatible bzip2
files, unless memory management takes over 2MB, which
sounds impropable.

> Options in 7-zip create archive dialog box are not
shared.

Only when specifying different archive formats. Using
different compression methods within 7z archive still involve a
lot of retyping.

[Igor Pavlov]

Logged In: YES
user_id=65083

> bzip2 manual, stated memory used for compression
is 400k
+ ( 8 x block size ), so 10MB will create incompatible
bzip2
files, unless memory management takes over 2MB,
which
sounds impropable.

400k + ( 8 x block size ) = 400K+7200K = 7600K
it's about 10M.