[Bug 16380] Loop devices act strangely in 2.6.35

[Bug 16380] Loop devices act strangely in 2.6.35

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https://bugzilla.kernel.org/show_bug.cgi?id=16380


Artem S. Tashkinov <t.artem@mailcity.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
          Component|Block Layer                 |ext2
         AssignedTo|axboe@kernel.dk             |fs_ext2@kernel-bugs.osdl.or
                   |                            |g
            Product|IO/Storage                  |File System




--- Comment #8 from Artem S. Tashkinov <t.artem@mailcity.com>  2011-03-22 22:02:54 ---
This is an ext2 fs bug.

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[Bug 16380] ext2 driver "hangs" for a few seconds when trying to write a lot of data to a freshly RW mounted FS

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https://bugzilla.kernel.org/show_bug.cgi?id=16380


Artem S. Tashkinov <t.artem@mailcity.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
            Summary|Loop devices act strangely  |ext2 driver "hangs" for a
                   |in 2.6.35                   |few seconds when trying to
                   |                            |write a lot of data to a
                   |                            |freshly RW mounted FS




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[Bug 16380] ext2/ext4 filesystem "hangs" for a few seconds when trying to write a lot of data to a freshly RW mounted FS

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https://bugzilla.kernel.org/show_bug.cgi?id=16380


Artem S. Tashkinov <t.artem@mailcity.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
          Component|ext2                        |ext4
         AssignedTo|fs_ext2@kernel-bugs.osdl.or |fs_ext4@kernel-bugs.osdl.or
                   |g                           |g
            Summary|ext2 driver "hangs" for a   |ext2/ext4 filesystem
                   |few seconds when trying to  |"hangs" for a few seconds
                   |write a lot of data to a    |when trying to write a lot
                   |freshly RW mounted FS       |of data to a freshly RW
                   |                            |mounted FS




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[Bug 16380] ext2/ext4 filesystem "hangs" for a few seconds when trying to write a lot of data to a freshly RW mounted FS

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https://bugzilla.kernel.org/show_bug.cgi?id=16380





--- Comment #9 from Artem S. Tashkinov <t.artem@mailcity.com>  2012-08-04 11:19:36 ---
It's reproducible with ext4fs as well under Linux 3.5.

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[Bug 16380] ext2/ext4 filesystem "hangs" for a few seconds when trying to write a lot of data to a freshly RW mounted FS

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https://bugzilla.kernel.org/show_bug.cgi?id=16380


Prashant Shah <pshah.mumbai@gmail.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |pshah.mumbai@gmail.com




--- Comment #10 from Prashant Shah <pshah.mumbai@gmail.com>  2012-08-04 11:53:19 ---
Didnt face this issue with 3.2.0-23-generic - 64 bit.

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[Bug 16380] ext2/ext4 filesystem "hangs" for a few seconds when trying to write a lot of data to a freshly RW mounted FS

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https://bugzilla.kernel.org/show_bug.cgi?id=16380





--- Comment #11 from Prashant Shah <pshah.mumbai@gmail.com>  2012-08-04 12:16:16 ---
(In reply to comment #10)
> Didnt face this issue with 3.2.0-23-generic - 64 bit.

Sorry I was able to reproduce it under 3.2.0-23-generic - 64 bit

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[Bug 16380] ext2/ext4 filesystem "hangs" for a few seconds when trying to write a lot of data to a freshly RW mounted FS

[bugzilla-daemon]

https://bugzilla.kernel.org/show_bug.cgi?id=16380


Artem S. Tashkinov <t.artem@mailcity.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
     Kernel Version|2.6.35                      |2.6.x 3.x
         Regression|Yes                         |No




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[Bug 16380] ext2/ext4 filesystem "hangs" for a few seconds when trying to write a lot of data to a freshly RW mounted FS

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https://bugzilla.kernel.org/show_bug.cgi?id=16380





--- Comment #12 from Theodore Tso <tytso@mit.edu>  2012-08-04 18:13:20 ---
I wasn't able to duplicate this with an underlying ext4 file system, with the
loop file system being ext2, using a 3.5 kernel on a system with 16G.

I tried both using a variety of small and medium sized files, as well as one
very large file, and I didn't see any unexplained stuttering in write
bandwidth.   (There were times when cp -r was clearly writing to the in-memory
page cache, and the writeback hadn't begone yet.   There were also times when
we couldn't do any more writing, because we were busy reading from the source
drive.  But it all looked pretty normal to me.)

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[Bug 16380] ext2/ext4 filesystem "hangs" for a few seconds when trying to write a lot of data to a freshly RW mounted FS

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https://bugzilla.kernel.org/show_bug.cgi?id=16380





--- Comment #13 from Theodore Tso <tytso@mit.edu>  2012-08-04 18:14:08 ---
BTW, I was using iostat 1 to measure I/O activity.  Call me old-school; I don't
exactly trust GUI tools.  :-)

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[Bug 16380] ext2/ext4 filesystem "hangs" for a few seconds when trying to write a lot of data to a freshly RW mounted FS

[bugzilla-daemon]

https://bugzilla.kernel.org/show_bug.cgi?id=16380





--- Comment #14 from Artem S. Tashkinov <t.artem@mailcity.com>  2012-08-04 18:51:24 ---
(In reply to comment #12)
> I wasn't able to duplicate this with an underlying ext4 file system, with the
> loop file system being ext2, using a 3.5 kernel on a system with 16G.
> 
> I tried both using a variety of small and medium sized files, as well as one
> very large file, and I didn't see any unexplained stuttering in write
> bandwidth.   (There were times when cp -r was clearly writing to the in-memory
> page cache, and the writeback hadn't begone yet.   There were also times when
> we couldn't do any more writing, because we were busy reading from the source
> drive.  But it all looked pretty normal to me.)

Hm, I cannot reproduce the problem with a loop device on top of tmpfs either,
but I've just checked my 250GB ext4 partition and the bug is still there:

When I try to copy a 3GB file (residing in tmpfs so it's all cached) to it,
there's a 4 (four) seconds delay before a single byte gets written to the
destination partition. After that all consequent files get written without any
delays.

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[Bug 16380] ext2/ext4 filesystem "hangs" for a few seconds when trying to write a lot of data to a freshly RW mounted FS

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https://bugzilla.kernel.org/show_bug.cgi?id=16380





--- Comment #15 from Theodore Tso <tytso@mit.edu>  2012-08-04 19:51:51 ---
The delay before the write starts is normal; that's encoded in how the
writeback code handles dirty pages.  We don't start writes the instant that
pages are dirtied in the buffer cache.

In any case, if you have arguments about how the writeback code makes its
choices, it's kinda of pointless to be complaining on a bug targetted at the
ext4 developers, since we start the writeback when the VM system asks us to
start write pages belonging to a particular inode....

See /usr/src/linux/Documentation/sysctl/vm.txt for more information.

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[Bug 16380] ext2/ext4 filesystem "hangs" for a few seconds when trying to write a lot of data to a freshly RW mounted FS

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https://bugzilla.kernel.org/show_bug.cgi?id=16380


Artem S. Tashkinov <t.artem@mailcity.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
         Resolution|                            |INVALID




--- Comment #16 from Artem S. Tashkinov <t.artem@mailcity.com>  2012-08-04 20:00:28 ---
(In reply to comment #15)
> The delay before the write starts is normal; that's encoded in how the
> writeback code handles dirty pages.  We don't start writes the instant that
> pages are dirtied in the buffer cache.
> 
> In any case, if you have arguments about how the writeback code makes its
> choices, it's kinda of pointless to be complaining on a bug targetted at the
> ext4 developers, since we start the writeback when the VM system asks us to
> start write pages belonging to a particular inode....
> 
> See /usr/src/linux/Documentation/sysctl/vm.txt for more information.

Thanks for letting us know, probably what I perceived as a regression in ext4fs
was in fact a change in the VM writeback code.

It's still weird and terribly counter intuitive as no other OS exhibit this
behavior, but now that I know why it happens I need to file a different bug
report.

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[Bug 16380] ext2/ext4 filesystem "hangs" for a few seconds when trying to write a lot of data to a freshly RW mounted FS

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https://bugzilla.kernel.org/show_bug.cgi?id=16380

--- Comment #17 from Artem S. Tashkinov <t.artem@mailcity.com> ---
Created attachment 166031
  --> https://bugzilla.kernel.org/attachment.cgi?id=166031&action=edit
Video demonstration

I'm NOT reopening this bug report. I'm merely adding a video clip which
pertains to it.

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