linux-kernel - Re: [PATCH 4/4] brd: Request from fdisk 4k alignment

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Message-ID: <20140807135110.GC9401@x2.net.home>
Date:	Thu, 7 Aug 2014 15:51:10 +0200
From:	Karel Zak <kzak@...hat.com>
To:	Boaz Harrosh <openosd@...il.com>
Cc:	Ross Zwisler <ross.zwisler@...ux.intel.com>,
	Boaz Harrosh <boaz@...xistor.com>,
	Jens Axboe <axboe@...nel.dk>,
	Matthew Wilcox <willy@...ux.intel.com>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>
Subject: Re: [PATCH 4/4] brd: Request from fdisk 4k alignment

On Thu, Aug 07, 2014 at 03:00:42PM +0200, Karel Zak wrote:
> On Thu, Aug 07, 2014 at 03:17:23PM +0300, Boaz Harrosh wrote:
> 
> > > Is there an error case that this patch fixes?  I've had page alignment checks
> > > in my PRD direct_access code forever, and I don't know if they've ever
> > > tripped.  
> > > 
> > 
> > Yes! as I said above fix fdisk. You never tripped on it because partitions never
> > worked and you never tried them. With current code fdisk is very trigger happy
> 
>  What do you mean with fdisk? which version?

Oh, I read all your email more carefully now, and if I good understand
the problem is what I/O limits the device provides to userspace rather
than with fdisk.

Anyway, I think you're right that alignment_offset is not the right
thing. It was introduced for backward compatibility with DOS-like
partitioning tools (~magical sector 63). I have doubts it's usable for
something else. For normal use-case should be enough to set proper
phy-sector size or min/optimal I/O limits (e.g. zram has all the
limits set to 4K(PAGE_SIZE)).

$ lsblk --topology /dev/zram0
NAME  ALIGNMENT MIN-IO OPT-IO PHY-SEC LOG-SEC ROTA SCHED RQ-SIZE  RA WSAME
zram0         0   4096   4096    4096    4096    0           128 128 0B

    Karel

-- 
 Karel Zak  <kzak@...hat.com>
 http://karelzak.blogspot.com
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