can subvolumes be specified as compressed?

can subvolumes be specified as compressed?

[Wang Shaoyan]

Hi.
I want a subvolume in root to be compressed, I try to do this:
# mkfs.btrfs /dev/sda8
# mount /dev/sda8 /home/usr/btrfs
# cd /home/usr/btrfs
# btrfs subvolume create mysubvolume
# mount -o compress,subvol=mysubvolume /dev/sda8 some_dir_in_root
but when I create a file in some_dir_in_root, the file isn't
compressed at all. why? How can I specify some subvolumes to be
compressed, and the others Not to be compressed in a single btrfs?

--
Wang Shaoyan

Re: can subvolumes be specified as compressed?

[Miao Xie]

On Thu, 29 Jul 2010 15:50:28 +0800, Wang Shaoyan wrote:
> Hi.
> I want a subvolume in root to be compressed, I try to do this:
> # mkfs.btrfs /dev/sda8
> # mount /dev/sda8 /home/usr/btrfs
> # cd /home/usr/btrfs
> # btrfs subvolume create mysubvolume
> # mount -o compress,subvol=mysubvolume /dev/sda8 some_dir_in_root
> but when I create a file in some_dir_in_root, the file isn't
> compressed at all. why? How can I specify some subvolumes to be
> compressed, and the others Not to be compressed in a single btrfs?

The method that you said is right, by this way, I can create a compressed file
on the specified subvolume.

I think your file has been compressed or is hard to be compressed, so
you found it isn't compressed at all. Just like we use a zip tool to compress
a compressed file, we will find the file isn't compressed at all.

Thanks
Miao

>
> --
> Wang Shaoyan
> --
> To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in
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>
>

Re: can subvolumes be specified as compressed?

[Wang Shaoyan]

A prerequisite is: when first mounted file system, not specify '-o
compress' options, only on mount subvolume
Below is my test:
# mkfs.btrfs /dev/sda8
# mount /dev/sda8 btrfs
# df
    Used: 56k
# btrfs subvolume create mysubvolume
# mkdir some_dir_in_root
# mount -o compress,subvol=3Dmysubvolume /dev/sda8 some_dir_in_root
# cd some_dir_in_root
# dd if=3D/dev/zero of=3Dbigfile count=3D2048 //create a all-0 file of =
1M byte
# df
    Used:1088k
we can find it is not compressed at all.

Another test:if we mount subvolume with "-o compress" first, then we
mount /dev/sda8 without "-o compress", and create a test file in
somewhere not in subvolume, we can find the file is compressed too.

2010/7/29 Miao Xie <miaox@cn.fujitsu.com>:
> On Thu, 29 Jul 2010 15:50:28 +0800, Wang Shaoyan wrote:
>>
>> Hi.
>> I want a subvolume in root to be compressed, I try to do this:
>> # mkfs.btrfs /dev/sda8
>> # mount /dev/sda8 /home/usr/btrfs
>> # cd /home/usr/btrfs
>> # btrfs subvolume create mysubvolume
>> # mount -o compress,subvol=3Dmysubvolume /dev/sda8 some_dir_in_root
>> but when I create a file in some_dir_in_root, the file isn't
>> compressed at all. why? How can I specify some subvolumes to be
>> compressed, and the others Not to be compressed in a single btrfs?
>
> The method that you said is right, by this way, I can create a compre=
ssed
> file
> on the specified subvolume.
>
> I think your file has been compressed or is hard to be compressed, so
> you found it isn't compressed at all. Just like we use a zip tool to
> compress
> a compressed file, we will find the file isn't compressed at all.
>
> Thanks
> Miao
>
>>
>> --
>> Wang Shaoyan
>> --
>> To unsubscribe from this list: send the line "unsubscribe linux-btrf=
s" in
>> the body of a message to majordomo@vger.kernel.org
>> More majordomo info at =A0http://vger.kernel.org/majordomo-info.html
>>
>>
>
>



--=20
Wang Shaoyan
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Re: can subvolumes be specified as compressed?

[Miao Xie]

On Thu, 29 Jul 2010 16:55:03 +0800, Wang Shaoyan wrote:
> A prerequisite is: when first mounted file system, not specify '-o
> compress' options, only on mount subvolume
> Below is my test:
> # mkfs.btrfs /dev/sda8
> # mount /dev/sda8 btrfs
> # df
>      Used: 56k
> # btrfs subvolume create mysubvolume
> # mkdir some_dir_in_root
> # mount -o compress,subvol=mysubvolume /dev/sda8 some_dir_in_root
> # cd some_dir_in_root
> # dd if=/dev/zero of=bigfile count=2048 //create a all-0 file of 1M byte
> # df
>      Used:1088k
> we can find it is not compressed at all.
>
> Another test:if we mount subvolume with "-o compress" first, then we
> mount /dev/sda8 without "-o compress", and create a test file in
> somewhere not in subvolume, we can find the file is compressed too.

Sorry! I made a mistake.

According to the implement of the btrfs, after we mount a btrfs filesystem
(include subvol), if we mount the same filesystem again or its subvol, the
kernel doesn't allocate a new super_block object, and use the super_block
object that allocated and initiated at the first mount. So the feature is also
the same as the filesystem mounted first.

Thanks
Miao

>
> 2010/7/29 Miao Xie<miaox@cn.fujitsu.com>:
>> On Thu, 29 Jul 2010 15:50:28 +0800, Wang Shaoyan wrote:
>>>
>>> Hi.
>>> I want a subvolume in root to be compressed, I try to do this:
>>> # mkfs.btrfs /dev/sda8
>>> # mount /dev/sda8 /home/usr/btrfs
>>> # cd /home/usr/btrfs
>>> # btrfs subvolume create mysubvolume
>>> # mount -o compress,subvol=mysubvolume /dev/sda8 some_dir_in_root
>>> but when I create a file in some_dir_in_root, the file isn't
>>> compressed at all. why? How can I specify some subvolumes to be
>>> compressed, and the others Not to be compressed in a single btrfs?
>>
>> The method that you said is right, by this way, I can create a compressed
>> file
>> on the specified subvolume.
>>
>> I think your file has been compressed or is hard to be compressed, so
>> you found it isn't compressed at all. Just like we use a zip tool to
>> compress
>> a compressed file, we will find the file isn't compressed at all.
>>
>> Thanks
>> Miao
>>
>>>
>>> --
>>> Wang Shaoyan
>>> --
>>> To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in
>>> the body of a message to majordomo@vger.kernel.org
>>> More majordomo info at  http://vger.kernel.org/majordomo-info.html
>>>
>>>
>>
>>
>
>
>

Re: can subvolumes be specified as compressed?

[Wang Shaoyan]

So, is there such a method: in a specified directory, the compression
feature is enabled, and the file in this directory will be compressed
automatically, while the others don't enable compression.

thanks.

2010/7/29 Wang Shaoyan <stufever@gmail.com>:
> A prerequisite is: when first mounted file system, not specify '-o
> compress' options, only on mount subvolume
> Below is my test:
> # mkfs.btrfs /dev/sda8
> # mount /dev/sda8 btrfs
> # df
> =A0 =A0Used: 56k
> # btrfs subvolume create mysubvolume
> # mkdir some_dir_in_root
> # mount -o compress,subvol=3Dmysubvolume /dev/sda8 some_dir_in_root
> # cd some_dir_in_root
> # dd if=3D/dev/zero of=3Dbigfile count=3D2048 //create a all-0 file o=
f 1M byte
> # df
> =A0 =A0Used:1088k
> we can find it is not compressed at all.
>
> Another test:if we mount subvolume with "-o compress" first, then we
> mount /dev/sda8 without "-o compress", and create a test file in
> somewhere not in subvolume, we can find the file is compressed too.
>
> 2010/7/29 Miao Xie <miaox@cn.fujitsu.com>:
>> On Thu, 29 Jul 2010 15:50:28 +0800, Wang Shaoyan wrote:
>>>
>>> Hi.
>>> I want a subvolume in root to be compressed, I try to do this:
>>> # mkfs.btrfs /dev/sda8
>>> # mount /dev/sda8 /home/usr/btrfs
>>> # cd /home/usr/btrfs
>>> # btrfs subvolume create mysubvolume
>>> # mount -o compress,subvol=3Dmysubvolume /dev/sda8 some_dir_in_root
>>> but when I create a file in some_dir_in_root, the file isn't
>>> compressed at all. why? How can I specify some subvolumes to be
>>> compressed, and the others Not to be compressed in a single btrfs?
>>
>> The method that you said is right, by this way, I can create a compr=
essed
>> file
>> on the specified subvolume.
>>
>> I think your file has been compressed or is hard to be compressed, s=
o
>> you found it isn't compressed at all. Just like we use a zip tool to
>> compress
>> a compressed file, we will find the file isn't compressed at all.
>>
>> Thanks
>> Miao
>>
>>>
>>> --
>>> Wang Shaoyan
>>> --
>>> To unsubscribe from this list: send the line "unsubscribe linux-btr=
fs" in
>>> the body of a message to majordomo@vger.kernel.org
>>> More majordomo info at =A0http://vger.kernel.org/majordomo-info.htm=
l
>>>
>>>
>>
>>
>
>
>
> --
> Wang Shaoyan
>



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Wang Shaoyan
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Re: can subvolumes be specified as compressed?

[Chris Ball]

Hi,

   > So, is there such a method: in a specified directory, the
   > compression feature is enabled, and the file in this directory
   > will be compressed automatically, while the others don't enable
   > compression.

I don't think so, but it wouldn't be difficult -- you'd just set a
per-inode flag on whether to compress, and if it's set on a directory
then it's inherited by the files in that directory.

I think the hold up has been that we've waited for a *generic*
cross-filesystem flag to appear, and it hasn't (or it has and I didn't
notice).  Does anyone have suggestions on how to break the deadlock
there?

- Chris.
-- 
Chris Ball   <cjb@laptop.org>
One Laptop Per Child

Re: can subvolumes be specified as compressed?

[Wang Shaoyan]

I don't know whether this demand, which specified some subvolumes or
directories to be compressed, conflict with the design of btrfs's
compression feature, may be they want to do this thing well, and
everyone will enable the compression feature, rather than throw the
decision to user to determine which should compress.

But now, if we can mount a subvolume and specify some mount options,
we should make these options work, rather than ignore.

2010/7/30 Chris Ball <cjb@laptop.org>:
> Hi,
>
> =A0 > So, is there such a method: in a specified directory, the
> =A0 > compression feature is enabled, and the file in this directory
> =A0 > will be compressed automatically, while the others don't enable
> =A0 > compression.
>
> I don't think so, but it wouldn't be difficult -- you'd just set a
> per-inode flag on whether to compress, and if it's set on a directory
> then it's inherited by the files in that directory.
>
> I think the hold up has been that we've waited for a *generic*
> cross-filesystem flag to appear, and it hasn't (or it has and I didn'=
t
> notice). =A0Does anyone have suggestions on how to break the deadlock
> there?
>
> - Chris.
> --
> Chris Ball =A0 <cjb@laptop.org>
> One Laptop Per Child
>



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Re: can subvolumes be specified as compressed?

[Chris Ball]

Hi,

   > I don't know whether this demand, which specified some subvolumes
   > or directories to be compressed, conflict with the design of
   > btrfs's compression feature, may be they want to do this thing
   > well, and everyone will enable the compression feature, rather
   > than throw the decision to user to determine which should
   > compress.

You can't tell whether you can compress data well until you've already
spent CPU compressing it, though.  Right now we have a useful metric
of "if parts of the file fail to compress, bail out on compressing the
rest of the file, both now and in the future"; but it's only a metric.
There are cases this fails on, such as a tar archive that starts out
with JPEGs and continues with large (compressible) text files.

I think the argument that we don't need to offer user control because
we can just do a good job all the time only works if it's actually
true that we can do a good job all the time, and I don't think that's
true here.  So, I still support user control of the per-inode flag.

(I agree about the subvol flags; I think being able to set quotas and
compression per-subvol is on the roadmap and just not done yet.)

-- 
Chris Ball   <cjb@laptop.org>
One Laptop Per Child