linux-ext4 - Re: ext4 not currently doing (much) multi-block allocation?

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Date:	Sun, 15 Feb 2009 13:37:14 -0600
From:	Eric Sandeen <sandeen@...hat.com>
To:	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>
CC:	Theodore Tso <tytso@....edu>, linux-ext4@...r.kernel.org
Subject: Re: ext4 not currently doing (much) multi-block allocation?

Aneesh Kumar K.V wrote:
> On Sun, Feb 15, 2009 at 08:36:18AM -0500, Theodore Tso wrote:
>> On Sun, Feb 15, 2009 at 04:35:28PM +0530, Aneesh Kumar K.V wrote:
>>> Here is how it works. During writepages we loop through the dirty pages
>>> and build largest contiguous block extent (mpage_add_bh_to_extent). Then we call
>>> mpage_da_map_blocks. mpage_da_map_blocks does the mutli block request.
>>> Once we have the blocks allocated we map these blocks to the pages. And
>>> then we writeback one page at a time using writepage callback.
>> mpage_da_map_blocks() calls mpd->get_block, which is set to
>> ext4_da_get_block_write(), which allocates a single block at a time
>> (max_blocks is set to bh->b_size >> inode->i_blkbits).
> 
> 
> That bh>b_size indicate multiple blocks. 

I never did like this overloading of a buffer head for this sort of
purpose, for this (sometimes confusing) reason, but it's done throughout
the kernel... :)

But, maybe a comment would be in order just to make it clear  "This is a
mapping BH" or something.

-Eric

> we do the below in mpage_add_bh_to_extent
> 
> 2024         if (logical == next && (bh->b_state & BH_FLAGS) == lbh->b_state) {
> 2025                 lbh->b_size += b_size;
> 2026                 return;
> 2027         }
> 
>> Put another way, where is the call to ext4_get_blocks_wrap() which
>> does the multi-block request?  I don't see it...
>>
> 

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