Hammer performance

[Matthew Dillon]

:Jasse Jansson wrote:
:
:> Doesn't this use of the dd command create a sparse file ???
:
:This is something I wanted to ask for quite some time: does hammer
:support sparse files?

    Yes.

:if I create an empty file, seek to 1M, and write one byte of data,
:will it waste 1M worth of real disk space or not?

    It won't.  It will write out a single 64K record at the 1M mark.

: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.

					-Matt
					Matthew Dillon 
					<dillon at backplane.com>