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

[Karel Zak <kzak@redhat.com>]

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@redhat.com>
 http://karelzak.blogspot.com