linux-ext4 - Re: How to fix up mballoc

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Date:	Thu, 23 Jul 2009 10:51:58 -0700
From:	Mingming Cao <cmm@...ibm.com>
To:	Theodore Tso <tytso@....edu>
CC:	Eric Sandeen <sandeen@...hat.com>,
	Andreas Dilger <adilger@....com>, linux-ext4@...r.kernel.org
Subject: Re: How to fix up mballoc

Theodore Tso wrote:
> So I started looking to see how we might be able to improve mballoc to
> avoid freespace fragmentation, and I came up with the following high
> level design.  Does this look sane?   Have I overlooked anything?
>
> 1) In ext4_mb_normalize_request(), if the inode that we are allocating
> does not have any open file descriptors for write (i.e., it's already
> closed and we're allocating via delalloc) _and_ the inode was
> previously opened with O_CREAT and without O_APPEND (checked via a
> flag in EXT4_I(inode)), then do not normalize the size to a power of
> two, but rather to the filesystem blocksize.
>
> The idea here is that we should be trying to find an exact fit, since
> most of the time (except for log files, which get appended; hence the
> O_CREAT && !O_APPEND test) once a file is written, that is probably
> the final size for the file.  So normalizing the size for the
> preallocation area to a power of two will be counterproductive for
> most files.
>
>   
I am trying to understand what user cases prefer normalize allocation 
request size? If they are uncommon cases, perhaps
we should disable the normalize the allocation size disabled by default, 
unless the apps opens the files with O_APPEND?
> 2) If the there has been less than X files opened in Y jiffies the
> parent directory (using the dentry path used to open the file), then
> do not set EXT4_MB_HINT_GROUP_ALLOC in ext4_mb_group_or_file().  We
> can simulate this for without creating this patch to test #1 by
> setting mb_stream_request to 0 (which should completely disable group
> preallocation).
>
> 						- Ted
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