linux-kernel - Re: blk_queue_hardsect

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Date:	Fri, 6 Jun 2008 11:18:07 +0100
From:	Alan Cox <alan@...rguk.ukuu.org.uk>
To:	Jens Axboe <jens.axboe@...cle.com>
Cc:	Anil kumar <anils_r@...oo.com>, linux-kernel@...r.kernel.org,
	linux-scsi@...r.kernel.org
Subject: Re: blk_queue_hardsect_size 520-byte sector

On Fri, 6 Jun 2008 09:54:33 +0200
Jens Axboe <jens.axboe@...cle.com> wrote:

> On Thu, Jun 05 2008, Anil kumar wrote:
> > Hi,
> > 
> > Can I call blk_queue_hardsect_size in a driver and set it to 520-byte
> > as hardware sector size.  How does the kernel and block layers take
> > the request, will it be in multiples of 520-bytes aligned or 512 bytes
> > aligned?
> 
> No, Linux only supports power-of-2 hardware block sizes I'm afraid.

And only some of those - which is why we can't support CP/M disks and
also early smartmedia stuff (128/256 byte/sector)

If you are trying to deal with 520 byte blocks that are 512 bytes file
system data + 8 bytes of meta data then use 512 byte block sizes and put
the meta-data somewhere else or provide a different way to access it  -
eg ATA uses 512 byte sectors for normal ATA but the sg_io() ioctl path
allows the issuing of commands like READ_LONG for specific cases where
the additional meta data is needed.

Alan
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