linux-ext4 - Re: EXT4 data block allocation alignment

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Date:	Wed, 11 Jun 2014 11:58:17 -0600
From:	Andreas Dilger <adilger@...ger.ca>
To:	Teng-Feng Yang <shinrairis@...il.com>
Cc:	linux-ext4@...r.kernel.org
Subject: Re: EXT4 data block allocation alignment

On Jun 11, 2014, at 5:44 AM, Teng-Feng Yang <shinrairis@...il.com> wrote:
> Recently I have been working on a small project which requires data
> blocks of each file to be aligned to 512KB/1MB boundary for
> performance reason. As far as I know, this can be accomplished by
> setting the stripe unit on XFS or enabling the "bigalloc" feature on
> EXT4. However, the "bigalloc" feature has not yet been stated to be
> stable in the latest release. So, this makes me wondering if I can
> achieve this block allocation alignment by setting the stride-size on
> EXT4. The document of mke2fs only says that this value may be used by
> the block allocator, so I am not quite sure what it can offer for me
> in this case.

In fact, 1MB alignment is already the default behaviour for ext4
extent-mapped allocations, though it isn't enforced for small files.
It is possible to specify the alignment with "-E stripe_width={blocks}"
(blocks=256 for 1MB and =128 for 512KB, assuming 4KB blocksize).

It might be possible to implement this alignment as required instead
of optional for data blocks, but that would need a change to the code.
It would still be more space efficient than bigalloc, since bigalloc
will allocate a full 512KB for each metadata block or directory.

Cheers, Andreas

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