Re: Allocating inodes (was: choosing the file system block size)

[Mouse <mouse%Rodents-Montreal.ORG@localhost>]

>> If you double the FS fragment and block size, you divide the number
>> of cylinder groups by 4
> I don't get that.

Here's the relevant quote from David's mail:

> the per-cylinder allocation maps are constrained to fit in a single
> fs block

Assuming this is the limit on cg size, you get one factor of two from
each block storing twice as much data, the other from each block
storing twice as much data.

Okay, that's a somewhat flippant phrasing, but it's true.  The single
block that holds the allocation map is twice as big and thus can map
twice as many blocks - but the blocks it's mapping are twice as large,
so the overall effect is that each CG can store four times as much
space.

Thus, slightly loosely put, doubling frag/block sizes means 1/4 the
number of cgs for a given filesystem size.

/~\ The ASCII                             Mouse
\ / Ribbon Campaign
 X  Against HTML                mouse%rodents-montreal.org@localhost
/ \ Email!           7D C8 61 52 5D E7 2D 39  4E F1 31 3E E8 B3 27 4B