2007-07-08 13:13:33

by Ninel Evol

[permalink] [raw]
Subject: inserting/deleting into/from the middle of large files?

'lo there, =)

having a DVB-S receiver running Linux (PPC) I found myself
wondering how to delete data from the middle of a large file
(stripping a recording of ads, for example - or messing
around in a virtual disk file, etc.). Currently, the common
way of doing this seems to be by copying the file (leaving a
part behind) and then deleting the original. Of course, on a
large file (say 12 GB) this can take an eternity; also you
can run into trouble if the filesystem is nearly full...

Is it me, or doesn't that make any sense?
Having a block-oriented filesystem, operations like this
should only take an instance.

So basically I'm looking for functions to:
- insert a chunk into a file
- delete a chunk from a file
- move a chunk from one file into another

All of the above would be very useful when dealing with
large data, such as DVB-recordings (Linux being the nr.1 OS
on those receivers, naturally:-).

Seeing a large file as a chain of blocks, making such an
operation on a block-sized basis should already be easy to
accomplish. However, if you want full support there should
be the possibility to insert "sparse blocks" (less content
than the usual) within a chain of blocks (including the

Is this already possible?
Would it be difficult to implement?

Think about it: instead of copying gigabytes with the
drive's heads clicking around - taking minutes to hours,
such operations could be performed in (milli)seconds.

I think that there should indeed be a standard (Posix?) for
providing such functionality. (One call could be to
determine if the filesystem supports those operations fast -
it could return a version for instance, 0 meaning that the
operations, although provided, will be slow.)

What is needed in the first place however, is a filesystem
supporting those operations - making it first choice for
VDRs running Linux. (The others would surely follow soon
after, at which point there should be the chance for
establishing a standard.)

Looking forward for some enlightenment... :-)

LC ([email protected])

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