[Unix-jun72] Anyone working on V1 mkfs?
Tim Newsham
newsham at lava.net
Thu May 1 22:08:47 EDT 2008
> On another note, I thought of taking V6 mkfs.c as a start-point to rewrite
> V1 mkfs. I'm trying to work out what f_m and f_n do:
>
> if(fsys[n] == 'r') {
> if(f == 'k') {
> f_n = 24; /* RK, why these numbers? */
> f_m = 3;
> }
> if(f == 'p') {
> f_n = 10; /* RP, why these numbers? */
> f_m = 4;
> }
> }
Looks like they stagger the insertion of blocks onto the free list
so that they're not adjacent. It first builds a table in adr[] of
the stagger offsets, then it frees blocks from the top linearly
until it hits a round number, then it allocates as many staggered
blocks as it can until finally returning to allocating the bottom
f_n blocks in a linear fashion again. For example if f_n is 10 and
f_m is 4, and igh is 100 and low is 20 you puts blocks on the
list in this order:
0, - always zero first
100 - linear at top
99, 95, 91, 97, 93, 98, 94, 90, 96, 92, ...
33, 38, 34, 30, 36, 32
29, 28, 27, ... 22, 21, 20 -- linear at the bottom
as to why they chose these particular constants for these disks,
I can only guess that they figured having blocks roughly four
apart made for good performance due to read and seek latencies?
> Finally, are we going to use RK03 or RK05, as the sizes are different?
The sources mention "rk03" several times and give 4872 decimal as
the maximum block number allowed (and also rf0 with 1024 as maximum
block number).
> Warren
Tim Newsham
http://www.thenewsh.com/~newsham/
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