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SubjectRe: [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

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