Use tar to append?

[Mike McCarty]

Mikkel L. Ellertson wrote:

> The problem is that the entire archive is compressed, and not the

[snip]

No, it is not. Please speak of what you know, rather than
what you conjecture.[1]

I ran a test in which I created a tar archive, not compressed,
but straight tar format. The size of the file was 3653601280 bytes.

I created another tar archive, also not compressed straight
tar format. The size of that file was 10240 bytes. It took
less than a second to create.

I then used "tar A" to append the tiny archive to the larger
one, and the run time was 2:20; that is two minutes twenty
seconds. During that time, my machine was approx. 80% wait state,
and approx. 16% system state, per top.

It may be that, due to the format of a tar file, tar is
architecturally constrained to do individual seeks for
every file contained in the archive, and that doing
thousands of seeks in this rather large file is necessarily
time consuming. This archive has 34619 files in it. I watched
the file system size using df, and found that it did not vary
during this process, so I conjecture that this was seeks
rather than copies.

> If you have the
> space, you would be better off creating an uncompressed archive of
> all the files you want to back up, and then compress and split the
> archive at the end.

That's what I've been doing during this experimentation.

[snip]

[1] I have no problem with conjectures so long as they are
stated as such for exploration, rather than posed as solutions.

Mike
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