Date: Fri, 31 Jan 2003 17:59:17 +0000 From: David Laight <david@l8s.co.uk> To: Steve Byan <stephen_byan@maxtor.com> Cc: freebsd-fs@FreeBSD.ORG, tech-kern@netbsd.org Subject: Re: DEV_B_SIZE Message-ID: <20030131175917.E1487@snowdrop.l8s.co.uk> In-Reply-To: <4912E0FE-3539-11D7-B26B-00306548867E@maxtor.com>; from stephen_byan@maxtor.com on Fri, Jan 31, 2003 at 11:30:18AM -0500 References: <4912E0FE-3539-11D7-B26B-00306548867E@maxtor.com>
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On Fri, Jan 31, 2003 at 11:30:18AM -0500, Steve Byan wrote: > There's a notion afoot in IDEMA to enlarge the underlying physical > block size of disks to 4096 bytes while keeping a 512-byte logical > block size for the interface. Unaligned accesses would involve either a > read-modify-write or some proprietary mechanism that provides > persistence without the latency cost of a read-modify-write. There probably ought to be a way of making the larger physical size visible to systems that are willing to support larger block sizes. That way misaligned transfers would be far less likely. One problem to consider is that disks are still partitioned on cylinder boundaries. This is largely historic but isn't this doen't actually make much sense, since the geometry almost certainly varies across the disk and has to be faked to fit the ATA CHS limits and (on PCs) the BIOS interface. However what it does mean is that a partition could easily not start on a 8 (512 byte) sector boundary. So misaligned transefers are likely even if the filesystem itself is using 4k blocks. On a PC the partitioning will typically have the first one starting in sector 63, and the others at multiple of 16065 sectors from the start of the disk). This doesn't bode well for getting any aligned transfer at all. David -- David Laight: david@l8s.co.uk To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-fs" in the body of the message
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