Adrian Irving-Beer wrote:
On Thu, Mar 24, 2005 at 12:42:46PM -0600, John Arbash Meinel wrote:
Remember, the tarball is compressed, so you do get a little bit of delta
compression even though there are 2 copies in there.
Negative on that, AFAICT...
% dd if=/dev/urandom of=file1 bs=1k count=1k
1024+0 records in
1024+0 records out
1048576 bytes transferred in 0.338545 seconds (3097302 bytes/sec)
% cp file1 file2
% ls -l file1 file2
-rw-r--r-- 1 wisq wisq 1048576 2005-03-24 18:50 file1
-rw-r--r-- 1 wisq wisq 1048576 2005-03-24 18:51 file2
% tar -zcf files.tar.gz file1 file2
% ls -l files.tar.gz
-rw-r--r-- 1 wisq wisq 2097810 2005-03-24 18:51 files.tar.gz
It does depend on the size of the file versus the size of the
compression window:
$ dd if=/dev/random of=file1 bs=1k count=10
10+0 records in
10+0 records out
$ cp file1 file2
$ tar czf files.tar.gz file1 file2
$ ls -l file1 file2 files.tar.gz
-rw-r--r-- 1 jameinel jameinel 10240 Mar 24 18:07 file1
-rw-r--r-- 1 jameinel jameinel 10240 Mar 24 18:07 file2
-rw-r--r-- 1 jameinel jameinel 10586 Mar 24 18:07 files.tar.gz
$ dd if=/dev/random of=file1 bs=1k count=100
100+0 records in
100+0 records out
$ cp file1 file2
$ tar czf files.tar.gz file1 file2
$ ls -l file1 file2 files.tar.gz
-rw-r--r-- 1 jameinel jameinel 102400 Mar 24 18:09 file1
-rw-r--r-- 1 jameinel jameinel 102400 Mar 24 18:09 file2
-rw-r--r-- 1 jameinel jameinel 205198 Mar 24 18:09 files.tar.gz
I don't know what the gzip window is, but the bzip2 window is 900k. (If
we used bzip2 instead of gzip, the above holds true up until > 500k files).
$ dd if=/dev/random of=file1 bs=1k count=100
100+0 records in
100+0 records out
$ cp file1 file2
$ tar cjf files.tar.bz2 file1 file2
$ ls -l file1 file2 files.tar.bz2
-rw-r--r-- 1 jameinel jameinel 102400 Mar 24 18:09 file1
-rw-r--r-- 1 jameinel jameinel 102400 Mar 24 18:09 file2
-rw-r--r-- 1 jameinel jameinel 128478 Mar 24 18:11 files.tar.bz2
$ dd if=/dev/random of=file1 bs=1k count=500
500+0 records in
500+0 records out
$ cp file1 file2
$ tar cjf files.tar.bz2 file1 file2
$ ls -l file1 file2 files.tar.bz2
-rw-r--r-- 1 jameinel jameinel 512000 Mar 24 18:13 file1
-rw-r--r-- 1 jameinel jameinel 512000 Mar 24 18:13 file2
-rw-r--r-- 1 jameinel jameinel 750903 Mar 24 18:13 files.tar.bz2
$ dd if=/dev/random of=file1 bs=1k count=1k
1024+0 records in
1024+0 records out
$ cp file1 file2
$ tar cjf files.tar.bz2 file1 file2
$ ls -l file1 file2 files.tar.bz2
-rw-r--r-- 1 jameinel jameinel 1048576 Mar 24 18:12 file1
-rw-r--r-- 1 jameinel jameinel 1048576 Mar 24 18:12 file2
-rw-r--r-- 1 jameinel jameinel 2106766 Mar 24 18:12 files.tar.bz2
So I would say your mostly right. Unless the files are below the size of
the compression window, then you get pretty good delta compression.
John
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