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

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Date:	Tue, 23 Sep 2008 17:13:46 -0600
From:	Andreas Dilger <adilger@....com>
To:	Frédéric Bohé <frederic.bohe@...l.net>
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

On Sep 22, 2008  11:32 +0200, Frďż˝dďż˝ric Bohďż˝ wrote:
> Le lundi 22 septembre 2008 Ă  14:17 +0530, Aneesh Kumar K.V a ĂŠcrit :
> > 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.

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.

Cheers, Andreas
--
Andreas Dilger
Sr. Staff Engineer, Lustre Group
Sun Microsystems of Canada, Inc.

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