linux-kernel - Re: [PATCH 0/2] Squashfs: add LZ4 compression

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Date:	Mon, 22 Jul 2013 14:38:28 +0100
From:	Phillip Lougher <phillip.lougher@...il.com>
To:	Gu Zheng <guz.fnst@...fujitsu.com>
Cc:	Phillip Lougher <phillip@...ashfs.org.uk>,
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	linux-embedded@...r.kernel.org
Subject: Re: [PATCH 0/2] Squashfs: add LZ4 compression

On 22 July 2013 07:04, Gu Zheng <guz.fnst@...fujitsu.com> wrote:
> On 07/22/2013 01:07 PM, Phillip Lougher wrote:
>
>> On 22 July 2013 04:05, Gu Zheng <guz.fnst@...fujitsu.com> wrote:
>>> Hi Phillip,
>>>         Have some tests been carried out to confirm that Squashfs really
>>> can get benefit from LZ4 compression, comparing with lzo?
>>
>> This seems to be a loaded question, in that it seems to be trying to
>> reopen the "why add lz4 when we already have lzo" debate all over
>> again.  As LZ4 has been merged to mainline, this appears to be a
>> question that has already been answered.
>
> No, they are different. LZ4 can be merged to mainline, because we can
> see the benefit(faster compressing speed under the enabled unaligned
> memory access) it brings to us comparing with lzo.

The users of  LZ4 that has been merged is for decompression of kernel
and initramfs, not compression.  So far I don't see any users of the
LZ4 compression code in the kernel.  So you're saying it was merged
for fast compression despite there being no users of it?

> But it's hard to say that it also really can bring benefit to Squashfs.

Prejudiced statement without any evidence, if that's your contention prove it.

>
>>
>> As far as Squashfs is concerned,  I believe it is important to give
>> people the choice of using LZ4 to compress Squashfs filesystems now
>> its been mainlined.  As far as expected benefits are concerned,
>> Squashfs' use in embedded systems is very similar to compressing
>> kernels and initramfs data, in that it tends to be used to compress
>> root filesystems.  As such the benefits of using LZ4 in Squashfs
>> should be broadly similar to using LZ4 to compress kernels and
>> initramfs data.  Ultimately it is up to people to experiment and
>> choose whatever compression is best for their systems.
>
> Yes, but the premise is that using LZ4 in Squashfs is better than lzo
> in at least one actual situation.
>

No.  All it takes is for LZ4 to be better/faster at decompressing the
kernel or initramfs to make it advantageous to use LZ4 for Squashfs,
thus avoiding the overhead of another decompressor in the kernel..

>>
>> People are welcome to try the patches out and report their findings.
>
> Let's look and see other guys' feedbacks, they are more persuasive.:)

Suit yourself, maybe when you start using evidence rather than
unsubstantiated assertions, you'll be more persuasive too.  So far all
I see is wind.

Phillip

>
> Regards,
> Gu
>
>>
>> Phillip
>>
>>>
>>> Thanks,
>>> Gu
>>>
>>> On 07/22/2013 10:21 AM, Phillip Lougher wrote:
>>>
>>>> Hi
>>>>
>>>> Now that LZ4 compression support is in 3.11-rc1, I have written the
>>>> following two patches for Squashfs to use it.
>>>>
>>>> Phillip Lougher (2):
>>>>   Squashfs: add LZ4 compression support
>>>>   Squashfs: Add LZ4 compression configuration option
>>>>
>>>>  Documentation/filesystems/squashfs.txt |    8 +-
>>>>  fs/squashfs/Kconfig                    |   15 +++
>>>>  fs/squashfs/Makefile                   |    1 +
>>>>  fs/squashfs/decompressor.c             |    7 ++
>>>>  fs/squashfs/decompressor.h             |    4 +
>>>>  fs/squashfs/lz4_wrapper.c              |  163 ++++++++++++++++++++++++++++++++
>>>>  fs/squashfs/squashfs_fs.h              |    1 +
>>>>  7 files changed, 195 insertions(+), 4 deletions(-)
>>>>  create mode 100644 fs/squashfs/lz4_wrapper.c
>>>>
>>>> These patches are also available in the git tree here:
>>>>
>>>> browse: https://git.kernel.org/cgit/linux/kernel/git/pkl/squashfs-lz4.git
>>>> git clone: git://git.kernel.org/pub/scm/linux/kernel/git/pkl/squashfs-lz4.git
>>>>
>>>> LZ4 support has (obviously) also been added to the squashfs-tools
>>>> (Mksquashfs and Unsquashfs).  This is available from the Squashfs-tools
>>>> git repository here:
>>>>
>>>> browse: https://git.kernel.org/cgit/fs/squashfs/squashfs-tools.git
>>>> git clone: git://git.kernel.org/pub/scm/fs/squashfs/squashfs-tools.git
>>>>
>>>> When building the squashfs-tools edit the Makefile to enable LZ4
>>>> support (by default it is disabled).
>>>>
>>>> LZ4 compression can be specified by using the -comp option, e.g.
>>>> % mksquashfs xxx img.sqsh -comp lz4
>>>>
>>>> The use of LZ4 high compression can be specified using -Xhc, e.g.
>>>>
>>>> % mksquashfs xxx img.sqsh -comp lz4 -Xhc
>>>>
>>>> Phillip
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>>>
>>>
>>
>
>
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