linux-kernel - Re: [PATCH 4/5] brd: Request from fdisk 4k alignment

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Message-ID: <yq1sihwb5qz.fsf@sermon.lab.mkp.net>
Date:	Thu, 06 Nov 2014 12:25:56 -0500
From:	"Martin K. Petersen" <martin.petersen@...cle.com>
To:	Boaz Harrosh <boaz@...xistor.com>
Cc:	"Martin K. Petersen" <martin.petersen@...cle.com>,
	Jens Axboe <axboe@...com>,
	Matthew Wilcox <willy@...ux.intel.com>,
	Dmitry Monakhov <dmonakhov@...nvz.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	Karel Zak <kzak@...hat.com>
Subject: Re: [PATCH 4/5] brd: Request from fdisk 4k alignment

>>>>> "Boaz" == Boaz Harrosh <boaz@...xistor.com> writes:

Boaz,

Boaz> Note that blk_queue_physical_block_size() also trashes io_min, but
Boaz> we can leave this one to be 512. io_min as opposed to
Boaz> physical_block_size will actually change Kernel behavior

>> Care to elaborate?

I wanted you to elaborate on how io_min changes kernel behavior.

>> Also, fdisk and parted should be aligning on 1MB by default
>> regardless of block size. So I'm also not sure why you need to trick
>> fdisk to align by setting the pbs.

Boaz> fdisk will not align partition-start on 1M, only the very
Boaz> first-sector, libgparted and cfdisk are as you say, but with fdisk
Boaz> if you put any odd numbered partition-size, the next start-sector
Boaz> will just be the sum, but with this patch it will offer a small
Boaz> hole and will align the partition-start on 4K.  (And even with
Boaz> this patch user can enter bad values)

It really sounds like either your fdisk is way too old or you are
running it in DOS compat mode.

I don't have a fundamental issue reporting pbs of 4K. But if you are
only doing it to force a certain partition alignment then it sounds like
a kernel fix for a userland problem.

I've CC:ed Karel who can comment on fdisk partition alignment issues.

-- 
Martin K. Petersen	Oracle Linux Engineering
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