linux-ext4 - Re: [PATCH 3/3] mke2fs: document bigalloc and cluster-size

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Date:	Tue, 15 Jan 2013 14:46:17 -0500
From:	Phillip Susi <psusi@...ntu.com>
To:	Theodore Ts'o <tytso@....edu>
CC:	Zheng Liu <gnehzuil.liu@...il.com>, linux-ext4@...r.kernel.org,
	Zheng Liu <wenqing.lz@...bao.com>
Subject: Re: [PATCH 3/3] mke2fs: document bigalloc and cluster-size

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On 1/15/2013 2:12 PM, Theodore Ts'o wrote:
> BTW, I used the following modified text:
> 
> bigalloc This  feature  enables clustered allocation, so that the
> unit of allocation is a power of two  number  of blocks.   That
> is,  each bit in the what had tradi‐ tionally been known as the
> block  allocation  bitmap now  indicates  whether  a cluster is in
> use or not, where a cluster is by default composed of 16 blocks. 
> This  feature  can  decrease the time spent on doing block
> allocation and brings  smaller  fragmentation, especially  for
> large files.  The size can be speci‐ fied using the -C option.
> 
> Warning: The bigalloc feature is still under  devel‐ opment,  and
> may  not  be fully supported with your kernel or may have various
> bugs.  Please see the web page
> http://ext4.wiki.kernel.org/index.php/Bigalloc for details.

Does this mean that a cluster is the minimum allocation unit, or can
two small files allocate different blocks in the same cluster, leaving
the cluster partially used?  If the former, then how is this different
than just using a larger block size?


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