linux-ext4 - Re: Proposed design for big allocation blocks for ext4

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Message-ID: <20110225190436.GZ2924@thunk.org>
Date:	Fri, 25 Feb 2011 14:04:36 -0500
From:	Ted Ts'o <tytso@....edu>
To:	Amir Goldstein <amir73il@...il.com>
Cc:	Andreas Dilger <adilger@...ger.ca>, linux-ext4@...r.kernel.org
Subject: Re: Proposed design for big allocation blocks for ext4

On Fri, Feb 25, 2011 at 08:05:43PM +0200, Amir Goldstein wrote:
> 
> I like your design. very KISS indeed.
> I am just wondering why should BIGALLOC be INCOMPAT and not RO_COMPAT?
> After all, ro mount doesn't allocate and RO_COMPAT features are so muc
>  nicer...

I can try to make it be RO_COMPAT, but one thing my design changes is
that a block group will contain 32768 allocation blocks; so assuming a
4k blocks, instead of a block group containing a maximum of 32,768 4k
blocks comprising 128 MB, a block group would now contain 32,768 1M
blocks, or 32 GiB, or 8,388,608 4k blocks.

I'm pretty sure that existing kernels have superblock sanity checks
that will barf if they see this.  Still, yeah, I can try allocating
this as a ROCOMPAT feature, and later on, if people really care, they
can patch older kernels so they won't freak out when they see a
BigAlloc file system and can thus successfully mount it read-only.

(Right now existing kernels will complain when s_blocks_per_group is
greater than blocksize*8.)

						- Ted
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