Date: Tue, 4 Apr 2000 07:10:47 +1000 (EST) From: Bruce Evans <bde@zeta.org.au> To: Poul-Henning Kamp <phk@critter.freebsd.dk> Cc: "Justin T. Gibbs" <gibbs@FreeBSD.org>, cvs-committers@FreeBSD.org, cvs-all@FreeBSD.org Subject: Re: cvs commit: src/sys/kern kern_mib.c vfs_bio.c src/sys/sys buf.h Message-ID: <Pine.BSF.4.21.0004040649300.1047-100000@alphplex.bde.org> In-Reply-To: <23424.954794021@critter.freebsd.dk>
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On Mon, 3 Apr 2000, Poul-Henning Kamp wrote:
> In message <Pine.BSF.4.21.0004040545020.977-100000@alphplex.bde.org>, Bruce Eva
> ns writes:
>
> >The disk layer already handles special sector sizes without adding much
> >overhead to the usual case. I hope it can do something similar for
> >b_offset. E.g., only one of { b_blkno, b_offset } needs to be valid.
> >Theat one could be maintained by all layers and the other one ignored
> >by all layers.
>
> Let me see if I read you right:
> a 64bit byte offset in bio->bio_offset
> no bio->bio_*blkno fields
> we retain buf->b_blkno
>
> is that it ?
Not quite. Something obviously has to set the disk-level block number
(currently bio_pblkno). It should be set from a valid block number or
offset (bio_blkno or bio_offset). There needs to be a validity bit or
out-of bounds values to indicate invalid fields (I prefer the bit).
Callers prefer bio_blkno if it is convenient and works (i.e., in most
cases, until 1TB disks become common).
bio_offset could be overlayed with bio_blkno to save space (this requires
a validity bit). The validity bit essentially gives a block size.
It is set when the block size is 1 and the full 64 bits of the offset
are valid, and clear when the block size is DEV_BSIZE and only 32 bits
of the offset are valid.
Bruce
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