What, exactly, are stat.st_blocks, statfs.f_bsize?

[Chris Torek]

In article <1991Feb25.205932.16587 at athena.mit.edu> jik at athena.mit.edu
(Jonathan I. Kamens) writes:
>... on a 4.3BSD system, the stat structure contains the st_blocks field,
>which tells the "actual number of blocks allocated."  Given that
>description, the question becomes, what exactly is a "block?"  There are two
>possible answers:
>
>1. The size specified by DEV_BSIZE.
>2. The size in the f_bsize field of the statfs structure of the filesystem on
>   which the file resides.

The answer is `none of the above'.

>Now, it seemed to me that f_bsize would be the logical choice,

No: f_bsize is the `block' size and not the `fragment' size under
4.3BSD-reno, i.e., typically 8K rather than 1K.  (SunOS and 4BSD are
different here; SunOS defines f_bsize as the fragment size.)

>The 4.3reno stat(2) man page goes even further; it describes st_blocks
>as "The actual number of blocks allocated for the file in 512-byte units."
>But that leaves me with another question -- is it DEV_BSIZE, or 512 bytes?

It is 512 bytes; it does not matter what DEV_BSIZE is.  Under 4.3tahoe
on the Tahoe, DEV_BSIZE was 1024; 4.3reno has no DEV_BSIZE at all (well,
it has one as a compatibility hack) and each disk's block size is a
property of that disk.

Note that there may be (probably are) some systems out there in which
st_blocks is in terms of 1 kbyte blocks; these should dwindle away,
but will probably leave a lingering stench. :-)
-- 
In-Real-Life: Chris Torek, Lawrence Berkeley Lab EE div (+1 415 486 5427)
Berkeley, CA		Domain:	torek at ee.lbl.gov