Alzheimer's Syndrome (a.k.a. disappearing inodes) analysis & test

[Henry Spencer]

In article <308 at stca77.stc.oz> peter at stca77.stc.oz (Peter Jeremy) writes:
>...on a
>clean file system the free inode list should be sorted, and therefore
>inodes should be allocated in inode order).

Well, do remember that there's no such thing as a "free inode list" with
all the free inodes of the filesystem on it.  What there is, is an in-core
list of up to circa 100 free inodes.  That list is only a hint to improve
performance; it's the inode itself that records whether it is allocated
or not.  If the in-core list fills up, Unix just throws away the extra
hints.  If the in-core list becomes empty, Unix searches the filesystem
for more free inodes to refill it.  "Searches" means just what you think
it does:  sequential search through the inodes looking for empties.  In
modern Unixes an attempt is made to start the search from a different
point each time, to avoid long delays.  There is plenty of room for bugs
and odd behavior in this scheme.

(NB:  SysV and BSD have each added their own wrinkles which change some of
the details of the above.)
-- 
The meek can have the Earth;    |    Henry Spencer at U of Toronto Zoology
the rest of us have other plans.|uunet!attcan!utzoo!henry henry at zoo.toronto.edu