Hammer performance

[Bakul Shah]

On Mon, 07 Jul 2008 10:10:29 PDT Matthew Dillon <dillon at apollo.backplane.com>  wrote:
> :How about if I write a 1M of 0 over an existing file, will I recover
> :the space?
> :
> :Cedric
> 
>     No, if you write out zeros it will write zeros to the disk.  It
>     will not make that area sparse again.  We don't want to have to
>     scan data buffers to see if they are all zero.  The door is open
>     for some other method of re-creating a hole in a file but there
>     is no standard UNIX way of doing it at the moment.

Freeing a block of zeroes would be nice - my qemu images
would thank you :-)  This can be done transparently by the FS
(no need for user visible changes).

It might be worth checking if a block is all zeroes (first
check if the first and last word are all zeroes and if so
check the rest).  Even if one out of a few thousand disk
writes can be avoided, this is worth it.

While I am here let me also suggest adding SEEK_HOLE and
SEEK_DATA tyoes of lseek(2) which allows a quicker navigation
of holey files -- no need to do anything heroic, just find
the first null block pointer for SEEK_HOLE and find the first
non-null block pointer for SEEK_DATA.