linux-ext4 - Re: [PATCH v2] ext4: fix initialization of UNINIT bitmap blocks

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Message-Id: <1222261048.3511.23.camel@frecb007923.frec.bull.fr>
Date:	Wed, 24 Sep 2008 14:57:28 +0200
From:	Frédéric Bohé <frederic.bohe@...l.net>
To:	Andreas Dilger <adilger@....com>
Cc:	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>,
	Theodore Tso <tytso@....edu>,
	"linux-ext4@...r.kernel.org" <linux-ext4@...r.kernel.org>
Subject: Re: [PATCH v2] ext4: fix initialization of UNINIT bitmap blocks

Le mardi 23 septembre 2008 à 17:13 -0600, Andreas Dilger a écrit :
> On Sep 22, 2008  11:32 +0200, Fr�d�ric Boh� wrote:
> > Le lundi 22 septembre 2008 à 14:17 +0530, Aneesh Kumar K.V a écrit :
> > > What you can do is make ext4_group_info generic for both mballoc and
> > > oldalloc. We can then add bg_flag to the in memory ext4_group_info
> > > that would indicate whether the group is initialized or not. Here
> > > initialized for an UNINIT_GROUP indicate we have done
> > > ext4_init_block_bitmap on the buffer_head. Then 
> > > instead of depending on the buffer_head uptodate flag we can check
> > > for the ext4_group_info bg_flags and decided whether the block/inode
> > > bitmap need to be initialized.
> > 
> > That makes sense ! I agree with you, we need an additional in-memory
> > flag to know whether buffers are initialized or not. Anyway, making
> > ext4_group_info generic will lead to unneeded memory consumption for
> > oldalloc. Maybe a simple independent bits array could do the trick. Is
> > there any advantage to re-use ext4_group_info ?
> 
> For ext4 I think 99% of users will use mballoc, and the reduction in code
> complexity is itself useful.  I don't think the in-memory overhead is very
> much, maybe 1 MB per TB of filesystem space.
> 

You are right ext4_group_info structure was not as big as I thought.
Do you mean that making ext4_group_info generic for both mballoc and
oldalloc will reduce the code complexity ?

> Also, if you are considering this approach (to initialize the in-memory
> bitmaps at mount time) they should be written to disk even if unused.
> Please also consider doing the inode table zeroing at the same time.
> This would allow uninit_bg to avoid doing it at mke2fs time.

In fact, I was not considering doing this at mount time, but it could be
a good approach.
Anyway, I don't understand why we should write bitmaps to disk after
that, and why we should zeroing the inode table.  Don't we end up with a
fast mkfs and a slow mount doing all the stuff older mkfs was doing ?
The UNINIT feature would become less interesting.

Regards,
Frederic



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