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Date:	Sat, 21 Jan 2012 10:14:10 +0800
From:	Robin Dong <hao.bigrat@...il.com>
To:	Andreas Dilger <adilger@...ger.ca>
Cc:	Bluflonalgul <bluflonalgul@...il.com>,
	Ext4 Developers List <linux-ext4@...r.kernel.org>,
	"Ted Ts'o" <tytso@....edu>
Subject: Re: Ext4 bigalloc and sparc ext3 16k blocksize

2012/1/21 Andreas Dilger <adilger@...ger.ca>:
> On 2012-01-20, at 4:20 AM, Bluflonalgul wrote:
>> Some (misleading?) article on kernelnewbies.org said the new Ext4
>> Bigalloc feature was about supporting block size up to 1MB.
>> I tried to use this feature to read an ext3 fs with 16k blocksize made
>> on a Linux Debian Sparc (NAS).
>> But I couldn't read such filesystem with the Linux 3.2 kernel on x86
>> PC... It fails to read fs structure (as it used to fail with previous
>> kernels).
>
> The bigalloc feature is not intended to be disk compatible with a
> large blocksize filesystem, or no "feature" would be needed at all
> besides increasing the blocksize in the superblock.
>
> What it is intended to handle is efficient block allocation for large
> file IO, by increasing the size of space allocation/tracking in the
> block bitmap, without breaking the kernel paradigm of keeping block
> size <= PAGE_SIZE.
>
> This gives many of the benefits of having a large blocksize without
> needing to change the whole kernel.
>
>> Could someone point me to some documentation, or give me some clues:
>> I'd like to understand what's wrong and if I can hope to read such fs
>> with Linux on x86 (natively, without fusefs tricks or additional
>> tools).
>
> There was some work done by Robin Dong (2011-11-18) that would get us
> most of the way to just handling large blocksize filesystems directly
> by the kernel.  This might be facilitated by denying mmap access to
> such filesystems, but for media/big data filesystems (as opposed to the
> root fs) this is probably not a serious limitation.
>
> I'm still interested to see a continuation of Robin's work, taking it
> to just be disk compatible with large blocksize, even if it is not
> possible to use mmap IO on such filesystems (always setting MNT_NOEXEC
> on systems where PAGE_SIZE < blocksize and not supplying f_op->mmap
> should work).

A great idea of extended version of ext4_extent was mentioned by Ted
(2011-11-19 http://www.spinics.net/lists/linux-ext4/msg28999.html)

I am happily to buy this story which might solve the concerns of
TAOBAO corp and Andreas as well.
Therefore I will send a RFC later to continue :)


>
> The reason that this is desirable is that it allows bypassing the 16TB
> file size limitations, and it also allows mounting filesystems from
> SPARC, PPC, and IA64 systems that were formatted in this manner and are
> getting old and need replacing.
>
> Cheers, Andreas
>
>
>
>
>
> --
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-- 
--
Best Regard
Robin Dong
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