Multipart containers

Recently, I wrote a response to a newsgroup question about spanning data blocks over multiple media**. That post refers to an earlier post with a detailed example in the "appendix" section. The advantage of using a container made from predefined data blocks is that cut/paste operations are not required. This is because the container is defined using a device mapper table. There are a couple of disadvantages. First, the size of the container must be determined in advance*, and there could be some wasted space when the last block is not full, etc. The other main disadvantage is the extra complexity that the technique introduces. Certainly, it is much simpler to work without introducing sub-blocks and work with one big file instead. Sometimes that is not possible- necessity demands a more complex solution. This technique still has a useful place for me, especially when data exceeds the size of one optical disc.

* While writing this, I realized that these containers are actually dynamically expandable if certain conditions are met. The first condition is that the filesystem used for the container must be expandable. I use XFS and it is dynamically expandable. The second condition is that data stored on the container must be compatible with expansion. If these conditions are met, then expanding the container could be as simple as allocating another block and adding another line to the device mapper table configuration.


Update: 2010-12-28
** Once again, Google decided not to archive my response on that thread. Here is the content:

>On Fri, 10 Dec 2010 21:12:03 +0000, Rui Maciel wrote:
>
> Is it possible to create a multipart gzip archive without resorting to
> the split/cat pair of commands?
>
>
> Thanks in advance,
> Rui Maciel
>
Almost anything is possible; whether it is practical is another matter.
If it is a "one of a kind" or rare event, then it may not be practical
to investigate other possibilities. That said, it was practical for me
to figure this out because I need to write in blocks which "span" across
optical media quite often.

I wrote about the technique here:
http://groups.google.com/group/comp.os.linux.misc/msg/6e34937952916a9a

Once you define a "container" you can put whatever you like into it
(simple data, compressed data, loopback filesystems, encrypted data,
etc.)