Subject: RE: DEV_B_SIZE
To: 'Steve Byan' <>
From: Jamey Kirby <>
List: tech-kern
Date: 01/31/2003 10:55:47
I have been a lurker for years and want to chime in.

Under Windows NT (all flavors), using a 4K sector size works fine. The
OS abstraction layers are very good and handling the alignment.

I wrote a virtual SCSI disk driver (ATA is presented as SCSI to the NT
OS kernel) and experimented with all sorts of sector sizes to see how
various software would handle it. I found no problems... However, I
myself have written test code in the past that assumes 512 byte sectors
rather than reading the sector size from the OS. Surly this code would

Jamey Kirby

-----Original Message-----
From: owner-freebsd-fs@FreeBSD.ORG [mailto:owner-freebsd-fs@FreeBSD.ORG]
On Behalf Of Steve Byan
Sent: Friday, January 31, 2003 10:45 AM
To: David Laight
Cc: freebsd-fs@FreeBSD.ORG;
Subject: Re: DEV_B_SIZE

On Friday, January 31, 2003, at 12:59  PM, David Laight wrote:

> 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.

Yes, of course. But I asked with respect to an issue other than 
> 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.

We understand that problem. It's just a performance issue. My concern 
is that even if we handwave the performance issues, there's an 
underlying semantic that would not be satisfied if we were to run 
existing software, unmodified, on a disk with an underlying 4K sector 

Steve Byan <>
Design Engineer
Maxtor Corp.
MS 1-3/E23
333 South Street
Shrewsbury, MA 01545
(508) 770-3414

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