linux-kernel - Re: BTRFS partition usage...

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Date:	Tue, 12 Feb 2008 16:04:52 +0100 (CET)
From:	Jan Engelhardt <jengelh@...putergmbh.de>
To:	Chris Mason <chris.mason@...cle.com>
cc:	David Miller <davem@...emloft.net>, linux-kernel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org, btrfs-devel@....oracle.com
Subject: Re: BTRFS partition usage...

>
On Feb 12 2008 09:35, Chris Mason wrote:
>>
>> and slap the bootloader into "MBR", just like on x86.
>> Or I am missing something..
>
>It was a request from hpa, and he clearly had something in mind.  He kindly 
>offered to review the disk format for bootloaders and other lower level 
>issues but I asked him to wait until I firm it up a bit.
>
>>From my point of view, 0 is a bad idea because it is very likely to conflict 
>with other things.  There are lots of things in the FS that need deep 
>thought,and the perfect system to fully use the first 64k of a 1TB filesystem 
>isn't quite at the top of my list right now ;)
>
>Regardless of offset, it is a good idea to mop up previous filesystems where 
>possible, and a very good idea to align things on some sector boundary.  Even 
>going 1MB in wouldn't be a horrible idea to align with erasure blocks on SSD.

I still don't like the idea of btrfs trying to be smarter than a user
who can partition up his system according to
	(a) his likes
	(b) system or hardware requirements or recommendations
to align the superblock to a specific location.

1MB alignment does not always mean 1MB alignment.
Sector 1 begins at 0x7e00 on x86.
And with the maximum CHS geometry (255/63), partitions begin
at 0x7e00+n*8225280 bytes, so the SB is unlikely to ever be on
a 1048576 boundary.
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