linux-ext4 - Re: [PATCH] Improve buffered streaming write ordering

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Date:	Tue, 7 Oct 2008 10:38:00 -0400
From:	Chuck Lever <chuck.lever@...cle.com>
To:	Peter Staubach <staubach@...hat.com>
Cc:	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>,
	Christoph Hellwig <hch@...radead.org>,
	Chris Mason <chris.mason@...cle.com>,
	Dave Chinner <david@...morbit.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	ext4 <linux-ext4@...r.kernel.org>
Subject: Re: [PATCH] Improve buffered streaming write ordering


On Oct 7, 2008, at Oct 7, 2008, 9:55 AM, Peter Staubach wrote:

> Aneesh Kumar K.V wrote:
>> On Tue, Oct 07, 2008 at 05:05:54AM -0400, Christoph Hellwig wrote:
>>
>>> On Tue, Oct 07, 2008 at 02:15:31PM +0530, Aneesh Kumar K.V wrote:
>>>
>>>> +static int ext4_write_cache_pages(struct address_space *mapping,
>>>> +		      struct writeback_control *wbc, writepage_t writepage,
>>>> +		      void *data)
>>>> +{
>>>>
>>> Looking at this functions the only difference is killing the
>>> writeback_index and range_start updates.  If they are bad why  
>>> would we
>>> only remove them from ext4?
>>>
>>
>> I am also not updating wbc->nr_to_write.
>>
>> ext4 delayed allocation writeback is bit tricky. It does
>>
>> a) Look at the dirty pages and build an in memory extent of  
>> contiguous
>> logical file blocks. If we use writecache_pages to do that it will
>> update nr_to_write, writeback_index etc during this stage.
>>
>> b) Request the block allocator for 'x' blocks. We get the value x  
>> from
>> step a.
>>
>> c) block allocator may return less than 'x' contiguous block. That  
>> would
>> mean the variables updated by write_cache_pages need to corrected.  
>> The
>> old code was doing that. Chris Mason suggested it would make it easy
>> to use a write_cache_pages which doesn't update the variable for  
>> ext4.
>>
>> I don't think other filesystem have this requirement.
>
> The NFS client can benefit from only writing pages in strictly
> ascending offset order.  The benefit comes from helping the
> server to do better allocations by not sending file data to the
> server in random order.

For the record, it would also help prevent the creation of temporary  
holes in O_APPEND files.

If an NFS client writes the front and back ends of a request before it  
writes the middle, other clients will see a temporary hole in that  
file.  Applications (especially simple ones like "tail") are often not  
prepared for the appearance of such holes.

Over a client crash, data integrity would improve if the client was  
less likely to create temporary holes in files.

> There is also an NFS server in the market which requires data
> to be sent in strict ascending offset order.  This sort of
> support would make interoperating with that server much easier.

-- 
Chuck Lever
chuck[dot]lever[at]oracle[dot]com
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