Reserving Space on Disk

[Martin Weitzel]

In article <13422 at ulysses.att.com> cjc at ulysses.att.com (Chris Calabrese[mav]) writes:
:In article <MOSS.90Jul15190502 at ibis.cs.umass.edu>, moss at cs.umass.edu (Eliot Moss) writes:
:> [ ... ]
:> 
:> char buf[ONE_K];
:> int i;
:> 
:> for (i = 0; i < FOUR_K; ++i)
:>   write (fd, buf, ONE_K);
:> [ ... ]
:
:BTW, while reading the various ways to write some large space on the
:disk which have been passing by, I thought I'd pass a related tidbit
:along.  On many machine architectures, byte copying from user space to
:kernel space is quite a bit faster if the buffer is word aligned.
:
:For this reason, it is better to use malloc to get your buffer than to
:allocate it off of the stack (as malloc always returns maximally
:aligned memory).  Of course, this assumes that you'll use the buffer
:more than once, as the time to malloc() is around the same order as
:the time to deal with the non-aligned bytes at the beginning and end
:of the buffer.

What is wrong with the following approach (at least on non-BSD-ish
file systems)?

	while file has not desired size
		lseek(2) from current position forward
		disk-block-size bytes minus 1 and write(2)
		one byte

IMHO this should fill the disk and avoids much copying from user-space.
-- 
Martin Weitzel, email: martin at mwtech.UUCP, voice: 49-(0)6151-6 56 83