linux-ext4 - Re: possible dev branch regression

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Message-Id: <6B9FD2E7-4884-484C-BEBB-3E210007B049@gmail.com>
Date:	Tue, 19 Mar 2013 00:54:51 +0800
From:	"gnehzuil.liu" <gnehzuil.liu@...il.com>
To:	Eric Sandeen <sandeen@...hat.com>
Cc:	Theodore Ts'o <tytso@....edu>, Eric Whitney <enwlinux@...il.com>,
	"linux-ext4@...r.kernel.org" <linux-ext4@...r.kernel.org>
Subject: Re: possible dev branch regression - xfstest 285/1k

Hi Eric,

在 2013-3-19，上午12:10，Eric Sandeen <sandeen@...hat.com> 写道：

> On 3/16/13 10:06 PM, Theodore Ts'o wrote:
>> On Sat, Mar 16, 2013 at 11:09:23PM +0800, Zheng Liu wrote:
>>> 
>>> I see what's going on.  First of all it isn't a bug. :-)  Please let me
>>> describe why it happens.
>>> 
>>> In this commit (4f42f80a8f), it tries to fix a bug that we never zero
>>> out an unwritten extent.  So after applied it, when an unwritten extent
>>> is converted, it could be zeroed out.  In xfstests #285 subtest 08 it
>>> preallocates an unwritten extent which is 4MB.  Then it writes some data
>>> at offset 10 * blocksize, which the length is one blocksize, and calles
>>> sync_file_range(2) to flush it.
>> 
>> Specifically, we are now honoring the default setting which sets the
>> max_zeroout_kb value to be 32.  With a 4k block file system, if we
>> were to zeroout the extent, we would have to zero out 40k, which is
>> greater than 32k, so resulting file after pwrite(fd, 4096, 40960)
>> looks like this:
>> 
>> % filefrag -v /u1/foo08 
>> Filesystem type is: ef53
>> File size of /u1/foo08 is 4194304 (1024 blocks of 4096 bytes)
>> ext:     logical_offset:        physical_offset: length:   expected: flags:
>>   0:        0..       9:    1852416..   1852425:     10:             unwritten
>>   1:       10..      10:    1852426..   1852426:      1:            
>>   2:       11..    1023:    1852427..   1853439:   1013:             unwritten,eof
>> /u1/foo08: 1 extent found
>> 
>> With a 1k block file system, we only need to zero out 10k, which is
>> less than 32k, and so after pwrite(fd, 1024, 10240), the file looks
>> like this:
>> 
>> % filefrag -v /mnt/foo08
>> Filesystem type is: ef53
>> File size of /mnt/foo08 is 4194304 (4096 blocks of 1024 bytes)
>> ext:     logical_offset:        physical_offset: length:   expected: flags:
>>   0:        0..      10:      81921..     81931:     11:            
>>   1:       11..    4095:      81932..     86016:   4085:             unwritten,eof
>> /mnt/foo08: 1 extent found
> 
> So the issue is just that the test is looking for actual holes
> in specific locations , but the fs chose to allocate zero-filled
> blocks instead?

Yes, it is.

> 
>> If we run src/seek_sanity_test by hand, we can make it happy by
>> setting the following configuration option before we run it:
>> 
>> echo 0 > /sys/fs/ext4/<dev>/extent_max_zeroout_kb
> 
> The test could do this too, right?
> 
> _need_to_be_root
> 
> and:
> 
> if [ "$FSTYP" == "ext4" ]; then
>    ORIG_ZEROOUT_KB=`cat /sys/fs/ext4/$TEST_DEV/extent_max_zeroout_kb`
>    echo 0 > /sys/fs/ext4/$TEST_DEV/extent_max_zeroout_kb
> fi
> 
> and put it back to default in _cleanup:
> 
>    echo $ORIG_ZEROOUT_KB > /sys/fs/ext4/$TEST_DEV/extent_max_zeroout_kb
> 
> That way we'd be testing seek hole correctness w/o being subject to
> the vagaries in allocator behavior.

Good idea.  I will try it.

Thanks,
                                                - Zheng--
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