Re: O_TRUNC problem on a full filesystem

[Stephen C. Tweedie]

Hi,

On Fri, May 25, 2001 at 10:24:49AM +1000, Andrew Morton wrote:

> For example, when we miss the goal block we search forward
> up to 63 blocks for a *single* free block, and use that.
> Perhaps we shouldn't?

The reasoning here is that it's much cheaper to go to a single block
which is very nearby than to be forced to use that single block later
on as part of some distant file once the disk becomes fuller.  It's a
sort of opportunistic fragmentation: if we can sneak in a disk
allocation that uses the awkward block without requiring a seek (and
in all likelihood coming out of the track buffer), then we reduce the
overall impact on performance of that isolated free block.

> And perhaps the search for eight contiguous free blocks
> is no longer appropriate to current disks.  32 may be better?

I've thought about that but today we're usually allocating in 4k
chunks rather than 1k so it's normally a 32k preallocation which we
get, anyway.

Cheers,
 Stephen