On 12/19/2012 05:54 PM, Dave Chinner wrote:
On Wed, Dec 19, 2012 at 05:17:45PM -0500, Joe Landman wrote:
[...]
Pointers appreciated. I am looking at the copy routines in
coreutils now, looking to see if we can increase its intelligence
somewhat w.r.t. sparse files.
Here's a good overview of the state of play:
http://www.linuxplumbersconf.org/2012/wp-content/uploads/2012/08/sparse-improvements-LPC-2012.pdf
I git cloned the coreutils to look at current state of the code, and saw
exactly what is represented on slide 14.
"Brute force – read each sector in full, before
skipping while writing the copy"
There has to be a better way.
And what you really want is a version of cp that supports these:
$ man lseek
....
Seeking file data and holes
Since version 3.1, Linux supports the following additional
values for whence:
SEEK_DATA
Adjust the file offset to the next location in the
file greater than or equal to offset containing data.
If offset points to data, then the file offset is set
to offset.
SEEK_HOLE
Adjust the file offset to the next hole in the file
greater than or equal to offset. If offset points
into the middle of a hole, then the file offset is
set to offset. If there is no hole past offset, then
the file offset is adjusted to the end of the file
(i.e., there is an implicit hole at the end of any
file).
.....
Something akin to this. Actually would like to be able to have it pull
bmap data so that reading over a file only reads populated extents, so
that anything that is not populated is nulled out by definition. I am
guessing that these are the abstraction above bmap type data?
I'm pretty sure coreutils support is in the pipeline right now....
Hopefully, but it doesn't appear to be in the repository I cloned. :(
Was thinking of hacking something up at a much higher level (cheating by
parsing bmap data and stuff like that).
Cheers,
Dave.
Thanks for the pointers. If I come up with anything marginally useful
prior to the SEEK_{HOLE,DATA} implementation, I'll update.
--
Joe
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