On 08/01/06 12:40, Rick C. Petty wrote:
On Tue, Aug 01, 2006 at 12:27:54PM -0500, Eric Anderson wrote:
Wouldn't this be incorrect for files that are really full of zeros? It
would turn them in to sparse files when they shouldn't be, correct? Is
that what happens with other tools?
Why is this bad? I'm not suggesting that the default implementation
should change but that this "-s" option be added to properly optimize
for sparse files. A sparse file is one which contains blocks of pure
zeros. My example wouldn't vary in how the destination file is read
but in how many blocks are allocated in the underlying file system.
It could possibly be bad if you have a real file (say a 10GB file,
partially filled with zeros - a disk image created with dd for
instance), and you use cp with something like -spR to recursively copy
all files. Your destination disk image would then be a sparse file, so
additional changes and modifications to the file (block allocations)
would fragment the image and make it slower. That would be unexpected
and probably go unnoticed for some time. I do see the usefulness in
this, but I think one needs to be careful to either clearly note in the
man page that it will make sparse files from *any* file containing a
string of zeros larger than the block size, or it needs to 'do the right
thing' and determine if it's sparse or not.
Eric
--
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Eric Anderson Sr. Systems Administrator Centaur Technology
Anything that works is better than anything that doesn't.
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