Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!ames!dftsrv!mimsy!chris
From: chris@mimsy.umd.edu (Chris Torek)
Newsgroups: comp.unix.wizards
Subject: Re: Re^2: 4BSD file system structure
Message-ID: <20893@mimsy.umd.edu>
Date: 23 Nov 89 00:40:12 GMT
References: <1727@neon.siesoft.co.uk>
Organization: U of Maryland, Dept. of Computer Science, Coll. Pk., MD 20742
Lines: 26

In article <1727@neon.siesoft.co.uk> duncan@helium.siesoft.co.uk
(Duncan McEwan) writes:
>....  Do the disk addresses kept in the inode and indirect blocks
>address blocks or fragments?

The things *in* ip->i_db[i] are in terms of fragments.  The values i
used to index into ip->i_db[] are in terms of blocks.  That is:

	ip->i_db[howmany(offset, blksize)]

is the fragment-wise block number of the disk-block-or-fragment
that holds the bytes from floor(offset/blksize)*blksize to
ceil(offset/blksize)*blksize-1.

More specifically, on an 8k/1k file system, if you want to find
the set of 1024-byte disk `blocks' that contain bytes 8192..16383
of a given file, you look at ip->i_db[1].  If this contains the
number 2000, those bytes are in the eight consecutive 1024-byte
fragments numbered 2000, 2001, 2002, ..., 2007.  If, however, the
file is only 10240 bytes long, then there are only two such 1024-byte
blocks, namely 2000 and 2001.

There are various implications here that I leave to the reader.
-- 
In-Real-Life: Chris Torek, Univ of MD Comp Sci Dept (+1 301 454 7163)
Domain:	chris@cs.umd.edu	Path:	uunet!mimsy!chris