Re: ext2 in a dos file/partition

Andries.Brouwer@cwi.nl
Fri, 8 Mar 1996 02:17:32 +0100


Werner Almesberger writes:

:: When I worked on vax/vms years ago, we had a thing called a
:: "virtual disk"; the file in which it lived was called a "container file".
:: You could mount the virtual disk, format it, etc. just as if it was
:: a real disk.

: While the loopback devices will let you do that, I have some doubts that
: it will help performance in the general case, because you now depend
: on the FAT cache in addition to the buffer cache. Not a good idea.

Well, it seems to me that the only thing you require from the
FAT file system is the bmap() function.
[One might even check that the container file is consecutive,
and in that case skip bmap() entirely.]

: Note that you can increase the FAT cache a bit, but you shouldn't make
: it too large, because the access time for all operations (lookup, add,
: and invalidate) is O(n). I don't know how much "too large" is, but it
: it would be really nice if somebody could run benchmarks to find the
: break-even point on a slow PC.

But it is easy to change this O(n) into O(log n) with a somewhat larger
constant, at least for lookup and add.

Andries