Date: Fri, 17 Dec 2010 12:09:55 +0000 From: Bruce Cran <bruce@cran.org.uk> To: Alexander Best <arundel@freebsd.org> Cc: freebsd-scsi@freebsd.org Subject: Re: "camcontrol identify ada0" and "diskinfo -v ada0" reporting different cylinders count Message-ID: <20101217120955.00005f5a@unknown> In-Reply-To: <20101217115526.GA20772@freebsd.org> References: <20101217115526.GA20772@freebsd.org>
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On Fri, 17 Dec 2010 11:55:26 +0000 Alexander Best <arundel@freebsd.org> wrote: > 484518 # Cylinders according to firmware. > 16 # Heads according to firmware. > 63 # Sectors according to firmware. > ... > cylinders 16383 > heads 16 > sectors/track 63 > > ...so how many cylinders does my hdd have? It almost certainly doesn't have 8 platters for a start, and neither does it have 63 sectors per track. CHS is obsolete - the ATA-7 specification has marked the fields as obsolete and all disks made in the last 10 (maybe even 15?) years have used LBA. camcontrol displays what the disk reports via the IDENTIFY command (I'm not sure if it's the "current" or default geometry it reports): ATA says that disks larger than can be represented using CHS should report having 16383 cylinders, which is needed for compatibility with BIOSes. diskinfo just appears to take the size of the disk, assume 16 heads and 63 sectors per track and put the rest into cylinders. There's another set of CHS values, and that's in geom: check for "fwheads" and "fwsectors" in "gpart list". See http://lists.freebsd.org/pipermail/svn-src-head/2010-December/023134.html for a (very!) detailed discussion of the various ways of calculating CHS values. Unfortunately no decision was reached so it looks like we're going to have a mismatch between camcontrol, diskinfo, geom, cam and ata for a while yet. -- Bruce Cran
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