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SubjectRe: [RFC] Heads up on sys_fallocate()
On Mon, Mar 05, 2007 at 07:15:33AM -0800, Ulrich Drepper wrote:
> Well, I'm sure the kernel can do better than the code we have in libc
> now. The kernel has access to the bitmasks which say which blocks have
> already been allocated. The libc code does not and we have to be very
> simple-minded and simply touch every block. And this means reading it
> and then writing it back. The kernel would know when the reading part
> is not necessary. Add to then the block granularity (we use f_bsize as
> returned from fstatfs but that's not the best value in some cases) and
> you have compelling data to have generic code in the kernel. Then libc
> implementation can then go away completely which is a good thing.

You have a very good point; indeed since we don't export an interface
which allows userspace to determine whether or not a block is in use,
that does mean a huge amount of churn in the page cache. So maybe it
would be worth doing in the kernel as a result, although the libc
implementation still wouldn't be able to go away for long time due to
the need to be backwards compatible with older kernels that didn't
have this support.


- Ted
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