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

[Zheng Liu <gnehzuil.liu@gmail.com>]

On Thu, Feb 21, 2013 at 03:56:51PM +0100, Lukáš Czerner wrote:
> 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 ?

I agree with you that we should forbid user to use bigalloc feature with
block size = 1k or 2k because I guess no one really use it, at least in
Taobao we always use bigalloc feature with block size = 4k.

FWIW, recently I am considering whether we could remove 'data=journal'
and 'data=writeback' mode.  'data=journal' mode hurts performance
dramatically.  Further, 'data=writeback' seems also useless, especially
after we have 'no journal' feature.  TBH, these modes are lack of tests.
Maybe this is a crazy idea in my mind.

Regards,
                                                - Zheng
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