Re: atomicity

Robert Minichino (
Mon, 7 Dec 1998 06:57:13 -0500 (EST)

> > ext2fs tries to grab linear chunks of disk and divides the disk into
> > groups to also help to maintain locality. The BSD ffs papers [McKusik et
> > describe this sort of stuff well.
> I am astonished to hear that ext2fs uses cyl groups.
> I was of the impression that these were an artefact of ffs, and really
> not useful anymore. With modern disks, chances are excellent that the
> specific 3D geometry the file system sees is an utter fiction, and
> that cylinder locality is thus more-or-less a meaningless concept.

The ext2fs notion of cylinder groups relies only on adjacency in terms of
linear sector numbers, disregarding the bogus logical geometry reported by
the disk. Even on modern disk drives, accessing blocks closest in linear
sector number is usually fastest, and the ext2fs notion of cylinder groups
exploits this. Locality of reference in the linear sense thus still applies.

So, when the writer tries to keep the file contained within one cylinder
group, it is contained within a small section of the disk when viewed
linearly. Granted because disks are not linear, the length of time to seek
from one linear block to another is not linear, but this does not hurt us as
we do not even attempt to take advantage of it.

Robert Minichino
Chief Engineer
Denarius Enterprises, Inc.

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