Date: Tue, 04 Jan 2000 10:20:58 +0100 From: Poul-Henning Kamp <phk@critter.freebsd.dk> To: doc@freebsd.org Subject: save this piece of wisdom for posterity Message-ID: <9293.946977658@critter.freebsd.dk>
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Date: Tue, 4 Jan 2000 20:15:01 +1100 (EST)
From: Bruce Evans <bde@zeta.org.au>
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To: Peter Jeremy <peter.jeremy@ALCATEL.COM.AU>
Cc: hardware@FreeBSD.ORG
Subject: Re: wanna buy an EIDE harddisk ... 5400 or 7200 for home use (noise)
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On Tue, 4 Jan 2000, Peter Jeremy wrote:
> SCSI went though it's `disks are too big to use' stage at ~1GB - the
> original SCSI DA read/write commands only allowed a 21-bit LBA. The
> next problem will occur at 2^32 blocks (about 2TB), which is still a
> few years off for bare disks, though it may be a problem for RAID
> controllers before then.
There are also some PC BIOS-related problems.
There are lots of critical sizes for ATA disks:
0.5284GB (C/H/S = 1024/16/63) (limit for MFM disks and old software)
8.4557GB (C/H/S = 1024/256/63) (1024-cylinder limit for old software)
8.4552GB (C/H/S = 16383/16/63) (the ATA standard was changed in 1997 to
require the default C/H/S for large drives
to be precisely 16383/16/63 instead of a
geometry that actually allows access to
the entire drive. In other words, the
firmware is required to be specially broken
to limit the damage caused by old software.
This broke non-broken software like the 1996
FreeBSD wd driver.)
33.8228GB (C/H/S = 65536/16/63) (the FreeBSD wd driver now converts from
C/H/S = 16363/16/63 to actual_C/16/63.
This is horribly broken when
actual_C > 65536. The hardware only
supports 16-bit cylinder numbers, and
writing to cylinder 65536 actually writes
to cylinder 0.)
136.9020GB (C/H/S = 65536/16/255) (limit of CHS addressing)
137.4389GB (LBA mode) (limit of 28-bit LBA addressing)
Notes: 1GB = 10^9 bytes. All sizes are rounded down. The sector size
is assumed to be 512.
Bruce
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