Date:	Thu, 21 Jun 2012 19:04:31 +0800
From:	Yongqiang Yang <xiaoqiangnk@...il.com>
To:	"linux-ext4@...r.kernel.org" <linux-ext4@...r.kernel.org>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
	Yongqiang Yang <xiaoqiangnk@...il.com>,
	Allison Henderson <achender@...ux.vnet.ibm.com>
Subject: Re: [RFC] ext4: add an io-tree to track block allocation

On Thu, Jun 21, 2012 at 5:46 PM, Zheng Liu <gnehzuil.liu@...il.com> wrote:
> Hi all,
>
> This year at ext4 workshop a new idea that calls io-tree is proposed to
> solve some problmes in ext4 [1].  I summarize the problems that are
> needed to solve by io-tree in here:
> 1. reserve quota calculation in bigalloc
> 2. simplify puch hole implementation
> 3. simplify fiemap implementation
> 4. SEEK_DATA/HOLE implementation
Actually, we can accelerate
 ext4_da_write_cache_pages by looking up extent status tree rather
than page cache.  This is one of aims of the original patch sets.

>
> Meanwhile with io-tree, some codes can be improved as following:
> 1. accelerate get_block functions
> 2. simplify uninitialized extent conversion
> 3. fine granularity locking (extent lock)
>
> I make a plan to implement io-tree that can be divided into three-steps.
> Now I describe it in detailed.
>
> * Step 1
> The following problems will be solved in this step:
> 1. reserve quota calculation in bigalloc
> 2. simplify puch hole implementation
> 3. simplify fiemap implementation
> 4. SEEK_DATA/HOLE implementation
>
> Currently a patch set has been submitted to the mailing list by
> Yongqiang and Allison, which called status extent tree, and it has
> simplified fiemap implementation.  But it only works when delay
In my memory  reserveing quota for bigalloc is also resolved in the
original patch sets.  Was it sent out?  If not, I can send the patch
to you if you need it:-)

> allocation is enabled.  I will pick up this work.  Now I have rebased
> this patch set to 3.5-rc3, and renamed it to extent status tree as
> Darrick advised.
>
> Next I will try to solve the above problems and make it run in
> nodelalloc mode.
>
> * Step 2
> To be improved:
> 1. accelerate get_block functions
> 2. simplify uninitialized extent conversion
IMHO ext4_da_write_cache_pages can be improved in this step.

Yongqiang.
>
> For the above improvements, a status member will be added in extent
> status tree to indicate the current status of this extent.  I think that
> the status includes dealloc, allocated, uninit, and hole.  Then we can
> let get_block functions to lookup extent status tree firstly to
> accelerate get_block.  Meanwhile uninitialized extent conversion can be
> modified to reduce lock contention of i_mutex.
>
> * Step 3
> To be done:
> 1. fine granularity locking (extent lock)
>
> Now in ext4 it does some operations with i_mutex locking.  After adding
> extent status tree, we can avoid to take this lock as much as possible.
> It seems that a new member needs to be added to indicate the type of
> locking.  We can take a range lock with shared or exclusive, and, when a
> range is locked, it cannot be merged by other processes and other types
> extent lock.
>
> Dave Chinner said that maybe range lock can be used in xfs too.  So I
> will try to implement a generic extent locking as much as possible after
> step 3.
>
> Please review this RFC, and any feedbacks are appreciated.  Thanks.
>
> In addition, I remember that at ext4 workshop Ted mentions that a big
> extent tree has been implemented to improve extent cache.  So we need to
> consider whether need to merge big extent tree and io-tree or not after
> both big extent tree and io-tree have been done.
>
> 1. http://www.spinics.net/lists/linux-ext4/msg31742.html
>
> Regards,
> Zheng



-- 
Best Wishes
Yongqiang Yang
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