linux-kernel - Re: [PATCH 2/4 v2] brd: Add getgeo to block ops

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Date:	Thu, 7 Aug 2014 19:20:32 +0100
From:	One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk>
To:	Boaz Harrosh <boaz@...xistor.com>
Cc:	Jens Axboe <axboe@...nel.dk>,
	Ross Zwisler <ross.zwisler@...ux.intel.com>,
	Matthew Wilcox <willy@...ux.intel.com>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>
Subject: Re: [PATCH 2/4 v2] brd: Add getgeo to block ops

On Thu, 07 Aug 2014 17:03:08 +0300
Boaz Harrosh <boaz@...xistor.com> wrote:

> From: Boaz Harrosh <boaz@...xistor.com>
> 
> Some programs like fdisk, require HDIO_GETGEO to work, which requires we
> implement getgeo.
> 
> We set all hd_geometry members to 1, because this way fdisk
> math will not try its crazy geometry math and get stuff totally wrong.

If you are running a new storage system for god sake don't use DOS
partitioning, use GPT or something sane.

> +static int brd_getgeo(struct block_device *bd, struct hd_geometry *geo)
> +{
> +	/* Just tell fdisk to get out of the way. The math here is so
> +	 * convoluted and does not make any sense at all. With all 1s
> +	 * The math just gets out of the way.
> +	 * NOTE: I was trying to get some values that will make fdisk
> +	 * Want to align first sector on 4K (like 8, 16, 20, ... sectors) but
> +	 * nothing worked, I searched the net the math is not your regular
> +	 * simple multiplication at all. If you managed to get these please
> +	 * fix here. For now we use 4k physical sectors for this
> +	 */
> +	geo->heads = 1;
> +	geo->sectors = 1;
> +	geo->cylinders = 1;
> +	return 0;

This is then going to blow up on your with some other tool. Fix fdisk
instead. Lying to apps generally ends up like children lying to parents -
the lie gets more complicated to keep up each case you find until it
breaks.

Alan
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