linux-ext4 - Re: [PATCH] ext4: fix overhead calculation in bigalloc filesystem (Re: ... )

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Date:	Thu, 21 Feb 2013 15:56:51 +0100 (CET)
From:	Lukáš Czerner <lczerner@...hat.com>
To:	Zheng Liu <gnehzuil.liu@...il.com>
cc:	Lukáš Czerner <lczerner@...hat.com>,
	linux-ext4@...r.kernel.org, "Theodore Ts'o" <tytso@....edu>
Subject: Re: [PATCH] ext4: fix overhead calculation in bigalloc filesystem
 (Re: ... )

On Thu, 21 Feb 2013, Zheng Liu wrote:

..snip..

> > > > >  	/*
> > > > >  	 * All of the blocks before first_data_block are overhead
> > > > >  	 */
> > > > > -	overhead = EXT4_B2C(sbi, le32_to_cpu(es->s_first_data_block));
> > > > > +	overhead = EXT4_NUM_B2C(sbi, le32_to_cpu(es->s_first_data_block));
> > > 
> > > ...except this. I do not think this is right because we do not skip
> > > the first cluster right ? We're still using it, but we can never use
> > > the block before es->s_first_data_block. Please correct me if I am
> > > wrong.
> 
> Yes, I think you are right.
> 
> > 
> > moreover we do not allow bigalloc file system with block size < 4k.
> 
> No, we allow user to use bigalloc with block size < 4k, such as:
> 
>   mkfs.ext4 -b 1024 -C 4096 -O bigalloc ${dev}
> 
> This command formats a bigalloc filesystem with blocksize = 1k and
> clustersize = 4k, at least in e2fsprogs 1.42.7 it works well.
> 

Ok, i was pretty sure that we do not allow that, it's good to know.
Also, does it make any sense ? I do not think so, and I would really
consider the fact that we allow that as a bug. We should not allow
that otherwise it unnecessarily extending the test matrix.

What people think about restricting bigalloc _only_ for 4k block
size file systems ?

Thanks!
-Lukas
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