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

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Message-ID: <AANLkTi=j2LCgORZPdusuCEvo55Bed6CVJp0yFwEy7dGa@mail.gmail.com>
Date:	Fri, 25 Feb 2011 23:24:33 +0200
From:	Amir Goldstein <amir73il@...il.com>
To:	"Ted Ts'o" <tytso@....edu>
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 9:04 PM, Ted Ts'o <tytso@....edu> wrote:
> 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.)
>

no problem. just rename s_blocks_per_group to s_bigblocks_per_group
to be compatible with old kernels.
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