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Date:	Mon, 18 May 2015 12:24:52 -0700
From:	"Darrick J. Wong" <darrick.wong@...cle.com>
To:	"Theodore Ts'o" <tytso@....edu>
Cc:	linux-ext4@...r.kernel.org
Subject: Re: [PATCH 11/14] libext2fs: use fallocate for creating journals and
 hugefiles

On Sat, May 16, 2015 at 11:39:25PM -0400, Theodore Ts'o wrote:
> On Wed, May 13, 2015 at 05:22:19PM -0700, Darrick J. Wong wrote:
> > Use the new fallocate API for creating the journal and the mk_hugefile
> > feature.
> > 
> > Signed-off-by: Darrick J. Wong <darrick.wong@...cle.com>
> 
> I tried applying patches 9-11, and I found a regression.  If you add
> the following stanza to /etc/mke2fs.conf:
> 
> 	hugefile = {
> 		features = extent,huge_file,flex_bg,uninit_bg,dir_nlink,extra_isize,^resize_inode,sparse_super2
> 		hash_alg = half_md4
> 		num_backup_sb = 0
> 		packed_meta_blocks = 1
> 		make_hugefiles = 1
> 		inode_ratio = 4194304
> 		hugefiles_dir = /store
> 		hugefiles_name = big-data
> 		hugefiles_digits = 0
> 		hugefiles_size = 0
> 		hugefiles_align = 256M
> 		num_hugefiles = 1
> 		zero_hugefiles = false
> 		flex_bg_size = 262144
> 	}
> 
> ... then "mke2fs -Fq -T hugefile /dev/sdXX" should create a file
> system with a single file /store/big-data that starts at offset 256M
> and consumes the rest of the space.  For example, try the commands
> 
> % time mke2fs -Fq -T hugefile /tmp/foo.img 8T
> % debugfs -R "extents /store/big-data" /tmp/foo.img
> 
> With this patch applied, the file /store/big-data is a zero-length
> file, instead of a very big file consuming the whole disk.

Oops.  I missed that subtlety; it's a pretty quick fix to make it
fallocate all the way to the end.  I also found a small bookkeeping error
that eliminates the churn in the test case expect files.

> Arguably there should have been a test so that this regression would
> be detected automatically.  I'll take care of adding it.
> 
> (BTW, note how quickly the file /store/big-data is created using the
> mk_hugefile code.  Although I understand the new fallocate code is
> more general, hopefully this generality doesn't cause performance
> regression in terms of the file system layout or CPU time required to
> create the big-data file.)

A lot of the complexity deals with figuring out if for a given hole we should
merely try to extent (or merge) the left and right extents.  For empty files,
it figures out that there is no left/right extent and simply cuts to the
alloc-range-and-map loop.  I noticed that it seemed to slow down maybe a
tenth of a second (out of 5) for a 4TB file; is that too much of a regression?

--D

> 
> > --- a/tests/r_32to64bit_meta/expect
> > +++ b/tests/r_32to64bit_meta/expect
> > @@ -35,8 +35,8 @@ Change in FS metadata:
> >   Inode count:              65536
> >   Block count:              524288
> >   Reserved block count:     26214
> > --Free blocks:              858
> > -+Free blocks:              852
> > +-Free blocks:              857
> > ++Free blocks:              851
> >   Free inodes:              65046
> >   First block:              1
> >   Block size:               1024
> 
> Why these changes?  This implies the new fallocate code isn't creating
> an extent tree that isn't quite as efficient as the original code?
> 
>    	       	    	  	   	     - Ted
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