Delayed writes on removed files

[Jan Edler]

In article <595 at taux01.UUCP> amos at taux01.UUCP (Amos Shapir) asks
if any unix system bothers to cancel pending delayed writes of disk blocks
belonging to files that are being removed.

The current production operating system for the NYU Ultracomputer prototype
(symunix 1) does this.  The buffers are also moved to the front of the
buffer cache free list, but in the opposite order of the coresponding
blocks on the disk block free list (so that the buffer most likely
to be reassigned is for the block least likely to be needed soon).

The reason I implemented this feature is that the hardware has fairly
poor i/o performance, and I was looking for ways to avoid unnecessary
i/o operations.  The only data I have on the effectiveness of this
technique is numbers like the following, collected over the last 12 days:

number of file blocks freed: 112277
number of those that were in the buffer cache: 29844
number of those with pending delayed write: 12727

The disk blocks are fixed size, 2K bytes.  The filesystem is close
to the v7 (or system V) design.
Subjectively, the improvement in overall performance due to this feature
is somewhat noticable, but not staggering.

Jan Edler
NYU Ultracomputer Project
edler at nyu.edu