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Date:	Thu, 7 Aug 2014 17:21:12 +0200
From:	Karel Zak <>
To:	Boaz Harrosh <>
Cc:	Boaz Harrosh <>,
	Ross Zwisler <>,
	Jens Axboe <>,
	Matthew Wilcox <>,
	linux-kernel <>,
	linux-fsdevel <>
Subject: Re: [PATCH 4/4] brd: Request from fdisk 4k alignment

On Thu, Aug 07, 2014 at 04:57:18PM +0300, Boaz Harrosh wrote:

> I was not saying that fdisk is wrong. I was saying that if my block driver
> was *not* exporting 4K physical sectors through limits.physical_block_size

 yep, sorry (it's probably bad idea to read emails and listen to talks on

> then fdisk would be happy to not align my partition start on 4k and would
> give me funny values like 34 for first sector which makes my device unusable
> because in direct_access() API we must absolutely have 4K aligned partitions.
> >> to miss-align my partitions. Depending on size maybe not the very first one but the
> >> consecutive ones easily.
> > 
> >  it would be nice to have usable bug report...
> > 
> Setting limits.physical_block_size = 4k; was the only way I found that could cause
> fdisk to default to 4k alignment.

 fdisk uses physical sector size or minimal I/O size (greater value wins)

> I was trying to play with the  heads, sectors, cylinders; values but none I tried

 don't play with CHS, that's waste of time and it's completely ignored
 by fdisk by default

> would cause an alignment of 4k, not even of the first partition start.
> Please advise what I can do?

 IMHO you're right with your patch (alignment offset is IMHO bad way).
 It's all (brd) about pages, is there any reason to use something else
 for I/O limits?

 It would be also nice to set minimal and optimal io size, zero values
 in this case means (for userspace) that the device does not provide
 any I/O information to system. It's normal for old hw disks and then
 we use some built-in defaults, but I don't see a reason to do the
 same for virtual devices. 


 Karel Zak  <>
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