linux-ext4 - Re: [RFC] delayed allocation for ext4

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Message-ID: <20061223033123.GL44411608@melbourne.sgi.com>
Date:	Sat, 23 Dec 2006 14:31:23 +1100
From:	David Chinner <dgc@....com>
To:	Alex Tomas <alex@...sterfs.com>
Cc:	linux-ext4@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC] delayed allocation for ext4

On Fri, Dec 22, 2006 at 11:20:08PM +0300, Alex Tomas wrote:
> 
> Good day,
> 
> probably the previous set of patches (including mballoc/lg)
> is too large. so, I reworked delayed allocation a bit so
> that it can be used on top of regular balloc, though it
> still can be used with extents-enabled files only.
> 
> this time series contains just 3 patches:
> 
>  - booked-page-flag.patch
>    adds PG_booked bit to page->flags. it's used in delayed
>    allocation to mark space is already reserved for page
>    (including possible metadata)

So that mean's we'll have 2 separate mechanisms for marking
pages as delalloc. XFS uses the BH_delay flag to indicate
that a buffer (block) attached to the page is using delalloc.

FWIW, how does this mechanism deal with block size < page size?
Don't you have to track delalloc on a block basis rather than
a page basis?

>  - ext4-block-reservation.patch
>    this is scalable free space management. every time we
>    delay allocation of some page, a space (including metadata)
>    should be reserved
> 
>  - ext4-delayed-allocation.patch
>    delayed allocation itself, enabled by "delalloc" mount option.
>    extents support is also required. currently it works only
>    with blocksize=pagesize.

Ah, that's why you can get away with a page flag - you've ignored
the partial page delay state problem. Any plans to use the
existing method in the future so we will be able to use ext4 delalloc
on machines with a page size larger than 4k?

Cheers,

Dave.
-- 
Dave Chinner
Principal Engineer
SGI Australian Software Group
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