How to remount btrfs without compression?

How to remount btrfs without compression?

[dima]

Hello,

Is there any possibility to remount a compressed btrfs without any 
compression at all?

Syslinux bootloader does not understand any btrfs compression and 
whenever I edit syslinux.cfg on my compressed / subvolume, the file 
becomes compressed and thus unreadable by syslinux on the next boot.

I tried to remount / without the 'compress' option (and edit 
syslinux.cfg in uncompressed state) and while the "mount" command would 
not show compression any more, I can see in the /proc/mounts that 
compression is still there and the file still gets compressed after 
editing. But there seem to be no mount option like compress=none or 
something.

The only workaround I found is to boot from a live CD mount / without 
any compression and re-save syslinux.cfg. Then it the file gets 
uncompressed.

Are there any other options except for this workaround to temporarily 
remount btrfs without compression?

thanks

Re: How to remount btrfs without compression?

[Martin Steigerwald]

Am Montag, 7. November 2011 schrieb dima:
> Hello,

Hi Dima,
=20
> Is there any possibility to remount a compressed btrfs without any
> compression at all?
>=20
> Syslinux bootloader does not understand any btrfs compression and
> whenever I edit syslinux.cfg on my compressed / subvolume, the file
> becomes compressed and thus unreadable by syslinux on the next boot.
>=20
> I tried to remount / without the 'compress' option (and edit
> syslinux.cfg in uncompressed state) and while the "mount" command wou=
ld
> not show compression any more, I can see in the /proc/mounts that
> compression is still there and the file still gets compressed after
> editing. But there seem to be no mount option like compress=3Dnone or
> something.
>=20
> The only workaround I found is to boot from a live CD mount / without
> any compression and re-save syslinux.cfg. Then it the file gets
> uncompressed.
>=20
> Are there any other options except for this workaround to temporarily
> remount btrfs without compression?

What does lsattr show on the file? Have you tried chattr -c on the file=
? It=20
might help to do a btrfs filesystem defrag on the file to remove=20
compression, cause I don=C2=B4t think chattr -c itself will uncompress =
it.

As far as I understand it is possible to individually set compression=20
on/off on single files. Although the global thing should work as well I=
MHO=20
as least when the file is rewritten.

Ciao,
--=20
Martin 'Helios' Steigerwald - http://www.Lichtvoll.de
GPG: 03B0 0D6C 0040 0710 4AFA  B82F 991B EAAC A599 84C7
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Re: How to remount btrfs without compression?

[dima]

On 11/07/2011 09:19 PM, Martin Steigerwald wrote:
> Am Montag, 7. November 2011 schrieb dima:
>> Hello,
>
> Hi Dima,
>
>> Is there any possibility to remount a compressed btrfs without any
>> compression at all?
>>
>> Syslinux bootloader does not understand any btrfs compression and
>> whenever I edit syslinux.cfg on my compressed / subvolume, the file
>> becomes compressed and thus unreadable by syslinux on the next boot.
>>
>> I tried to remount / without the 'compress' option (and edit
>> syslinux.cfg in uncompressed state) and while the "mount" command wo=
uld
>> not show compression any more, I can see in the /proc/mounts that
>> compression is still there and the file still gets compressed after
>> editing. But there seem to be no mount option like compress=3Dnone o=
r
>> something.
>>
>> The only workaround I found is to boot from a live CD mount / withou=
t
>> any compression and re-save syslinux.cfg. Then it the file gets
>> uncompressed.
>>
>> Are there any other options except for this workaround to temporaril=
y
>> remount btrfs without compression?
>
> What does lsattr show on the file? Have you tried chattr -c on the fi=
le? It
> might help to do a btrfs filesystem defrag on the file to remove
> compression, cause I don=C2=B4t think chattr -c itself will uncompres=
s it.

Hi Martin,
Thanks for your reply.
Yes, I did check out lsattr. It shows that no flags are set. Setting=20
chattr +c then chattr -c and re-saving file has no effect either.

I also tried to defragment the file itself and the directory where it=20
was in without setting -c but it would not have any effect because / is=
=20
mounted with compression.

> As far as I understand it is possible to individually set compression
> on/off on single files.

Could not find how to turn it off though.

thanks
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Re: How to remount btrfs without compression?

[Eric Griffith]

On 11/7/2011 7:55 PM, dima wrote:
> On 11/07/2011 09:19 PM, Martin Steigerwald wrote:
>> Am Montag, 7. November 2011 schrieb dima:
>>> Hello,
>>
>> Hi Dima,
>>
>>> Is there any possibility to remount a compressed btrfs without any
>>> compression at all?
>>>
>>> Syslinux bootloader does not understand any btrfs compression and
>>> whenever I edit syslinux.cfg on my compressed / subvolume, the file
>>> becomes compressed and thus unreadable by syslinux on the next boot=
=2E
>>>
>>> I tried to remount / without the 'compress' option (and edit
>>> syslinux.cfg in uncompressed state) and while the "mount" command w=
ould
>>> not show compression any more, I can see in the /proc/mounts that
>>> compression is still there and the file still gets compressed after
>>> editing. But there seem to be no mount option like compress=3Dnone =
or
>>> something.
>>>
>>> The only workaround I found is to boot from a live CD mount / witho=
ut
>>> any compression and re-save syslinux.cfg. Then it the file gets
>>> uncompressed.
>>>
>>> Are there any other options except for this workaround to temporari=
ly
>>> remount btrfs without compression?
>>
>> What does lsattr show on the file? Have you tried chattr -c on the
>> file? It
>> might help to do a btrfs filesystem defrag on the file to remove
>> compression, cause I don=C2=B4t think chattr -c itself will uncompre=
ss it.
>
> Hi Martin,
> Thanks for your reply.
> Yes, I did check out lsattr. It shows that no flags are set. Setting
> chattr +c then chattr -c and re-saving file has no effect either.
>
> I also tried to defragment the file itself and the directory where it
> was in without setting -c but it would not have any effect because / =
is
> mounted with compression.
>
>> As far as I understand it is possible to individually set compressio=
n
>> on/off on single files.
>
> Could not find how to turn it off though.
>
> thanks
> --
> 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


Hey Dima, I know if you install a system without the compress flag=20
already done, you can force BTRFS to compress everything post-install b=
y=20
teling it to "rebalance" the filesystem. I'm under Win7 right now so I=20
dont know the exact command. Check btrfs --help for that.

I dont know if it works in reverse, but you can definitely try it. Edit=
=20
your fstab, remove the compress flag, reboot. Tell btrfs to rebalance=20
the system, reboot again. And I -THINK- that'll decompress all the file=
s
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Re: How to remount btrfs without compression?

[Fajar A. Nugraha]

On Tue, Nov 8, 2011 at 8:06 AM, Eric Griffith <egriffith92@gmail.com> wrote:
> Edit your
> fstab, remove the compress flag, reboot. Tell btrfs to rebalance the system,
> reboot again. And I -THINK- that'll decompress all the files

I think the original question was how to force uncompressed mode,
whether specific to a file or to a whole filesystem, without having to
reboot :)

AFAIK there's no way to do that.

-- 
Fajar

Re: How to remount btrfs without compression?

[Eric Griffith]

On 11/7/2011 8:52 PM, Fajar A. Nugraha wrote:
> On Tue, Nov 8, 2011 at 8:06 AM, Eric Griffith<egriffith92@gmail.com>  wrote:
>> Edit your
>> fstab, remove the compress flag, reboot. Tell btrfs to rebalance the system,
>> reboot again. And I -THINK- that'll decompress all the files
>
> I think the original question was how to force uncompressed mode,
> whether specific to a file or to a whole filesystem, without having to
> reboot :)
>
> AFAIK there's no way to do that.
>

Whoops! Misunderstood the question haha. Yeah, as far as decompressing 
just a single file; from what I've read, thats impossible.

Re: How to remount btrfs without compression?

[dima]

On 11/08/2011 10:54 AM, Eric Griffith wrote:
> On 11/7/2011 8:52 PM, Fajar A. Nugraha wrote:
>> On Tue, Nov 8, 2011 at 8:06 AM, Eric Griffith<egriffith92@gmail.com>
>> wrote:
>>> Edit your
>>> fstab, remove the compress flag, reboot. Tell btrfs to rebalance the
>>> system,
>>> reboot again. And I -THINK- that'll decompress all the files
>>
>> I think the original question was how to force uncompressed mode,
>> whether specific to a file or to a whole filesystem, without having to
>> reboot :)
>>
>> AFAIK there's no way to do that.
>>
>
> Whoops! Misunderstood the question haha. Yeah, as far as decompressing
> just a single file; from what I've read, thats impossible.


Eric, Fajar,
Thanks. Understood.

Yes, it is possible to remove the compress flag from fstab, reboot and 
even do not do any defragmentation/rebalancing - just re-save the file 
and it will be saved uncompressed. This works. But only with reboot...

Re: How to remount btrfs without compression?

[Chris Mason]

On Tue, Nov 08, 2011 at 11:00:42AM +0900, dima wrote:
> On 11/08/2011 10:54 AM, Eric Griffith wrote:
> >On 11/7/2011 8:52 PM, Fajar A. Nugraha wrote:
> >>On Tue, Nov 8, 2011 at 8:06 AM, Eric Griffith<egriffith92@gmail.com>
> >>wrote:
> >>>Edit your
> >>>fstab, remove the compress flag, reboot. Tell btrfs to rebalance the
> >>>system,
> >>>reboot again. And I -THINK- that'll decompress all the files
> >>
> >>I think the original question was how to force uncompressed mode,
> >>whether specific to a file or to a whole filesystem, without having to
> >>reboot :)
> >>
> >>AFAIK there's no way to do that.
> >>
> >
> >Whoops! Misunderstood the question haha. Yeah, as far as decompressing
> >just a single file; from what I've read, thats impossible.
> 
> 
> Eric, Fajar,
> Thanks. Understood.
> 
> Yes, it is possible to remove the compress flag from fstab, reboot
> and even do not do any defragmentation/rebalancing - just re-save
> the file and it will be saved uncompressed. This works. But only
> with reboot...

chattr -c on the file should work (followed by defrag or rewriting the
file).  I just retested and it seems to be broken right now.

I'll track it down.

-chris

Re: How to remount btrfs without compression?

[Chris Mason]

On Tue, Nov 08, 2011 at 10:01:51AM -0500, Chris Mason wrote:
> On Tue, Nov 08, 2011 at 11:00:42AM +0900, dima wrote:
> > On 11/08/2011 10:54 AM, Eric Griffith wrote:
> > >On 11/7/2011 8:52 PM, Fajar A. Nugraha wrote:
> > >>On Tue, Nov 8, 2011 at 8:06 AM, Eric Griffith<egriffith92@gmail.com>
> > >>wrote:
> > >>>Edit your
> > >>>fstab, remove the compress flag, reboot. Tell btrfs to rebalance the
> > >>>system,
> > >>>reboot again. And I -THINK- that'll decompress all the files
> > >>
> > >>I think the original question was how to force uncompressed mode,
> > >>whether specific to a file or to a whole filesystem, without having to
> > >>reboot :)
> > >>
> > >>AFAIK there's no way to do that.
> > >>
> > >
> > >Whoops! Misunderstood the question haha. Yeah, as far as decompressing
> > >just a single file; from what I've read, thats impossible.
> > 
> > 
> > Eric, Fajar,
> > Thanks. Understood.
> > 
> > Yes, it is possible to remove the compress flag from fstab, reboot
> > and even do not do any defragmentation/rebalancing - just re-save
> > the file and it will be saved uncompressed. This works. But only
> > with reboot...
> 
> chattr -c on the file should work (followed by defrag or rewriting the
> file).  I just retested and it seems to be broken right now.
> 
> I'll track it down.

Ok, I had forgotten.  chattr -c clears the compression flag bug doesn't
set the no compress flag.  We looks like we need to patch chattr for
this.

-chris

Re: How to remount btrfs without compression?

[dima]

On 11/09/2011 12:12 AM, Chris Mason wrote:
> On Tue, Nov 08, 2011 at 10:01:51AM -0500, Chris Mason wrote:
>> On Tue, Nov 08, 2011 at 11:00:42AM +0900, dima wrote:
>>> On 11/08/2011 10:54 AM, Eric Griffith wrote:
>>>> On 11/7/2011 8:52 PM, Fajar A. Nugraha wrote:
>>>>> On Tue, Nov 8, 2011 at 8:06 AM, Eric Griffith<egriffith92@gmail.com>
>>>>> wrote:
>>>>>> Edit your
>>>>>> fstab, remove the compress flag, reboot. Tell btrfs to rebalance the
>>>>>> system,
>>>>>> reboot again. And I -THINK- that'll decompress all the files
>>>>>
>>>>> I think the original question was how to force uncompressed mode,
>>>>> whether specific to a file or to a whole filesystem, without having to
>>>>> reboot :)
>>>>>
>>>>> AFAIK there's no way to do that.
>>>>>
>>>>
>>>> Whoops! Misunderstood the question haha. Yeah, as far as decompressing
>>>> just a single file; from what I've read, thats impossible.
>>>
>>>
>>> Eric, Fajar,
>>> Thanks. Understood.
>>>
>>> Yes, it is possible to remove the compress flag from fstab, reboot
>>> and even do not do any defragmentation/rebalancing - just re-save
>>> the file and it will be saved uncompressed. This works. But only
>>> with reboot...
>>
>> chattr -c on the file should work (followed by defrag or rewriting the
>> file).  I just retested and it seems to be broken right now.
>>
>> I'll track it down.
>
> Ok, I had forgotten.  chattr -c clears the compression flag bug doesn't
> set the no compress flag.  We looks like we need to patch chattr for
> this.
>
> -chris
>


Just for the record - I could find a solution thanks to the btrfs wiki 
being online again. In Gotchas it says

mount -o nodatacow also disables compression

and indeed it does. Remounting with this option and re-saving the file 
makes it uncompressed. However, I could not find how to remount the 
filesystem afterwards without nodatacow.

~dima

Re: How to remount btrfs without compression?

[Lubos Kolouch]

dima, Wed, 09 Nov 2011 10:01:13 +0900:

> On 11/09/2011 12:12 AM, Chris Mason wrote:
>> On Tue, Nov 08, 2011 at 10:01:51AM -0500, Chris Mason wrote:
>>> On Tue, Nov 08, 2011 at 11:00:42AM +0900, dima wrote:
>>>> On 11/08/2011 10:54 AM, Eric Griffith wrote:
>>>>> On 11/7/2011 8:52 PM, Fajar A. Nugraha wrote:
>>>>>> On Tue, Nov 8, 2011 at 8:06 AM, Eric
>>>>>> Griffith<egriffith92@gmail.com> wrote:
>>>>>>> Edit your fstab, remove the compress flag, reboot. Tell btrfs to
>>>>>>> rebalance the system,
>>>>>>> reboot again. And I -THINK- that'll decompress all the files
>>>>>>
>>>>>> I think the original question was how to force uncompressed mode,
>>>>>> whether specific to a file or to a whole filesystem, without having
>>>>>> to reboot :)
>>>>>>
>>>>>> AFAIK there's no way to do that.
>>>>>>
>>>>>>
>>>>> Whoops! Misunderstood the question haha. Yeah, as far as
>>>>> decompressing just a single file; from what I've read, thats
>>>>> impossible.
>>>>
>>>>
>>>> Eric, Fajar,
>>>> Thanks. Understood.
>>>>
>>>> Yes, it is possible to remove the compress flag from fstab, reboot
>>>> and even do not do any defragmentation/rebalancing - just re-save the
>>>> file and it will be saved uncompressed. This works. But only with
>>>> reboot...
>>>
>>> chattr -c on the file should work (followed by defrag or rewriting the
>>> file).  I just retested and it seems to be broken right now.
>>>
>>> I'll track it down.
>>
>> Ok, I had forgotten.  chattr -c clears the compression flag bug doesn't
>> set the no compress flag.  We looks like we need to patch chattr for
>> this.
>>
>> -chris
>>
>>
> 
> Just for the record - I could find a solution thanks to the btrfs wiki
> being online again. In Gotchas it says
> 
> mount -o nodatacow also disables compression
> 
> and indeed it does. Remounting with this option and re-saving the file
> makes it uncompressed. However, I could not find how to remount the
> filesystem afterwards without nodatacow.
> 
> ~dima

Sorry for possibly OT question - when I have historical btrfs system
mounted with zlib compression,

can I remount it with lzo ? What will happen? Will the COW be broken
and the files taking duplicate space? Or will the Universe explode and
be replaced with something even more bizzare?

Thank you 

Lubos

Re: How to remount btrfs without compression?

[Dmitry Olenin]

On 11/09/2011 04:48 PM, Lubos Kolouch wrote:
> Sorry for possibly OT question - when I have historical btrfs system
> mounted with zlib compression,
>
> can I remount it with lzo ? What will happen? Will the COW be broken
> and the files taking duplicate space? Or will the Universe explode and
> be replaced with something even more bizzare?

Hello Lubos
If you have a kernel that supports lzo (don't quite remember when it got 
in), why can't you?
Absolutely nothing will happen, and only the new/updated files will be 
with lzo compression. You can remount on the fly switching b/w the two 
compression options without any problems.

Re: How to remount btrfs without compression?

[Fajar A. Nugraha]

On Wed, Nov 9, 2011 at 2:48 PM, Lubos Kolouch <lubos.kolouch@gmail.com> wrote:
> Sorry for possibly OT question - when I have historical btrfs system
> mounted with zlib compression,
>
> can I remount it with lzo ?

yes

> What will happen? Will the COW be broken
> and the files taking duplicate space? Or will the Universe explode and
> be replaced with something even more bizzare?

New written block/extents will use lzo compression (if it's
compressible, or if it's mounted with compress-force). Old, unmodified
block/extents will remain unchanged, using zlib or uncompressed.

-- 
Fajar

Re: How to remount btrfs without compression?

[Chris Mason]

On Wed, Nov 09, 2011 at 03:04:01PM +0700, Fajar A. Nugraha wrote:
> On Wed, Nov 9, 2011 at 2:48 PM, Lubos Kolouch <lubos.kolouch@gmail.com> wrote:
> > Sorry for possibly OT question - when I have historical btrfs system
> > mounted with zlib compression,
> >
> > can I remount it with lzo ?
> 
> yes
> 
> > What will happen? Will the COW be broken
> > and the files taking duplicate space? Or will the Universe explode and
> > be replaced with something even more bizzare?
> 
> New written block/extents will use lzo compression (if it's
> compressible, or if it's mounted with compress-force). Old, unmodified
> block/extents will remain unchanged, using zlib or uncompressed.

This is correct, your old stuff will be zlib and your new stuff lzo.
The compression type is recorded on a per-extent basis and the global
switch just decides what type of compression to use for new writes.

-chris

Re: How to remount btrfs without compression?

[David Sterba]

[-- Attachment #1: Type: text/plain, Size: 2193 bytes --]

On Wed, Nov 09, 2011 at 10:01:13AM +0900, dima wrote:
> Just for the record - I could find a solution thanks to the btrfs wiki 
> being online again. In Gotchas it says
> 
> mount -o nodatacow also disables compression
> 
> and indeed it does. Remounting with this option and re-saving the file 
> makes it uncompressed. However, I could not find how to remount the 
> filesystem afterwards without nodatacow.

I saw this mentioned on irc today (that nodatacow diasables
compression). There is a way how to turn off compression on a file --
with help of the NOCOW _file_ attribute, ie. you don't have to remount.

* create the file, compression enabled
* set NOCOW (with the attached single-purpose nocow.c utility)
* btrfs fi defrag the_file

Make sure you have enough free space for the uncompressed file size. You
can compare the extent layout before and after the defrag with
"filefrag -v" .

I did a test first, here's what I saw with a ~60M file (filled with
zeros, suitable for compression):

# filefrag -v zerofile2
Filesystem type is: 9123683e
File size of zerofile2 is 69927424 (17073 blocks, blocksize 4096)
 ext logical physical expected length flags
   0       0     2700              32
   1      32     2942     2731     32
   2      64     2943     2973     32
   3      96     2944     2974     32
   4     128     2945     2975     32
...
 533   17039     2641     2671     32
 534   17071     2643     2672      2 eof
zerofile2: 535 extents found

# btrfs fi defrag zerofile2

# filefrag -v zerofile2
Filesystem type is: 9123683e
File size of zerofile2 is 69927424 (17073 blocks, blocksize 4096)
 ext logical physical expected length flags
   0       0    20992            2107
   1    2107    23552    23098   1581
   2    3688    16400    25132   1185
   3    4873    12410    17584    889
   4    5762    14762    13298    667
   5    6429    15881    15428    500
   6    6929    17585    16380    751
   7    7680    23099    18335    375
   8    8055    25133    23473    376
   9    8431     3072    25508   2160
  10   10591     5632     5231   1620
  11   12211     8192     7251   2431
  12   14642    18432    10622   2431 eof
zerofile2: 13 extents found


david

[-- Attachment #2: nocow.c --]
[-- Type: text/plain, Size: 1220 bytes --]

#include <fcntl.h>
#include <stdlib.h>
#include <sys/ioctl.h>
#include <string.h>

#include <linux/types.h>
#include <stdio.h>
#include <errno.h>

#ifndef FS_IOC_SETFLAGS
#define FS_IOC_SETFLAGS                 _IOW('f', 2, long)
#warning defining SETFLAGS locally
#endif

#ifndef FS_IOC_GETFLAGS
#define FS_IOC_GETFLAGS                 _IOR('f', 1, long)
#warning defining GETFLAGS locally
#endif

#ifndef FS_NOCOW_FL
#define FS_NOCOW_FL                     0x00800000 /* Do not cow file */
#endif

int main(int argc, char **argv)
{
        int fd;
        int r;
	long flags;

	if (argc < 2) {
		printf("usage: %s file\n", argv[0]);
		exit(1);
	}

        fd = open(argv[1], O_RDONLY);
	if (fd == -1) {
		perror("open()");
		return 1;
	}
	printf("GETFLAGS ioctl 0x%x\n", FS_IOC_GETFLAGS);
	printf("SETFLAGS ioctl 0x%x\n", FS_IOC_SETFLAGS);

	r = ioctl(fd, FS_IOC_GETFLAGS, &flags);
	if (r == -1) {
		perror("ioctl(GETFLAGS)");
		return 1;
	} else {
		printf("file flags: 0x%lx\n", flags);
	}

	printf("Set NOCOW flag for %s\n", argv[1]);
	flags |= FS_NOCOW_FL;
        r = ioctl(fd, FS_IOC_SETFLAGS, &flags);
	printf("ioctl returned: %d\n", r);
	if (r == -1) {
		perror("ioctl()");
		return 1;
	}

        return 0;
}

Re: How to remount btrfs without compression?

[dima]

On 11/10/2011 09:11 AM, David Sterba wrote:
> On Wed, Nov 09, 2011 at 10:01:13AM +0900, dima wrote:
>> Just for the record - I could find a solution thanks to the btrfs wiki
>> being online again. In Gotchas it says
>>
>> mount -o nodatacow also disables compression
>>
>> and indeed it does. Remounting with this option and re-saving the file
>> makes it uncompressed. However, I could not find how to remount the
>> filesystem afterwards without nodatacow.
>
> I saw this mentioned on irc today (that nodatacow diasables
> compression). There is a way how to turn off compression on a file --
> with help of the NOCOW _file_ attribute, ie. you don't have to remount.
>
> * create the file, compression enabled
> * set NOCOW (with the attached single-purpose nocow.c utility)
> * btrfs fi defrag the_file
>
> Make sure you have enough free space for the uncompressed file size. You
> can compare the extent layout before and after the defrag with
> "filefrag -v" .


Hello David,

Thank you, I will try it out tonight. Is there any way to see if nocow 
attribute was set on a particular file, and is there any way to unset it?

thanks
~dima

Re: How to remount btrfs without compression?

[Lubos Kolouch]

Dmitry Olenin, Wed, 09 Nov 2011 17:03:39 +0900:

> On 11/09/2011 04:48 PM, Lubos Kolouch wrote:
>> Sorry for possibly OT question - when I have historical btrfs system
>> mounted with zlib compression,
>>
>> can I remount it with lzo ? What will happen? Will the COW be broken
>> and the files taking duplicate space? Or will the Universe explode and
>> be replaced with something even more bizzare?
> 
> Hello Lubos If you have a kernel that supports lzo (don't quite remember
> when it got in), why can't you?
> Absolutely nothing will happen, and only the new/updated files will be
> with lzo compression. You can remount on the fly switching b/w the two
> compression options without any problems.

Hello Dmitry,

By the way, this is interesting question to me - I mounted the filesystem
with -o compress=lzo, in dmesg showed btrfs: use lzo compression,

but - the lzo module was not loaded (not shown in lsmod - and yes, I have
it as a module).

When I do modprobe lzo, it shows there.

Isn't it a bit strange? So btrfs is using lzo module that was not loaded?
(and says so in the dmesg output)?

Thank you

Lubos

Re: How to remount btrfs without compression?

[Dmitry Olenin]

On 11/10/2011 03:57 PM, Lubos Kolouch wrote:
> Dmitry Olenin, Wed, 09 Nov 2011 17:03:39 +0900:
>
>> On 11/09/2011 04:48 PM, Lubos Kolouch wrote:
>>> Sorry for possibly OT question - when I have historical btrfs system
>>> mounted with zlib compression,
>>>
>>> can I remount it with lzo ? What will happen? Will the COW be broken
>>> and the files taking duplicate space? Or will the Universe explode and
>>> be replaced with something even more bizzare?
>>
>> Hello Lubos If you have a kernel that supports lzo (don't quite remember
>> when it got in), why can't you?
>> Absolutely nothing will happen, and only the new/updated files will be
>> with lzo compression. You can remount on the fly switching b/w the two
>> compression options without any problems.
>
> Hello Dmitry,
>
> By the way, this is interesting question to me - I mounted the filesystem
> with -o compress=lzo, in dmesg showed btrfs: use lzo compression,
>
> but - the lzo module was not loaded (not shown in lsmod - and yes, I have
> it as a module).
>
> When I do modprobe lzo, it shows there.
>
> Isn't it a bit strange? So btrfs is using lzo module that was not loaded?
> (and says so in the dmesg output)?

Hello Lubos
Yes, I was wondering about this too when I started doing lzo.
The lzo module you load most likely has nothing to do with btrfs. This 
is the only explanation. I would think that lzo for btrfs is built into 
kernel. Unfortunately I don't have my .config with me at the moment to 
check it.

Re: How to remount btrfs without compression?

[dima]

On 11/10/2011 11:23 AM, dima wrote:
> On 11/10/2011 09:11 AM, David Sterba wrote:
>> On Wed, Nov 09, 2011 at 10:01:13AM +0900, dima wrote:
>>> Just for the record - I could find a solution thanks to the btrfs wiki
>>> being online again. In Gotchas it says
>>>
>>> mount -o nodatacow also disables compression
>>>
>>> and indeed it does. Remounting with this option and re-saving the file
>>> makes it uncompressed. However, I could not find how to remount the
>>> filesystem afterwards without nodatacow.
>>
>> I saw this mentioned on irc today (that nodatacow diasables
>> compression). There is a way how to turn off compression on a file --
>> with help of the NOCOW _file_ attribute, ie. you don't have to remount.
>>
>> * create the file, compression enabled
>> * set NOCOW (with the attached single-purpose nocow.c utility)
>> * btrfs fi defrag the_file
>>
>> Make sure you have enough free space for the uncompressed file size. You
>> can compare the extent layout before and after the defrag with
>> "filefrag -v" .
>
>
> Hello David,
>
> Thank you, I will try it out tonight. Is there any way to see if nocow
> attribute was set on a particular file, and is there any way to unset it?


Hi David
I tried the nocow utility. It worked.
First I made sure to re-save the syslinux.cfg file while btrfs was 
mounted with lzo to ensure that it is compressed, then I set the NOCOW 
flag with your utility. Then I checked fragmentation of the file and 
since it was a small one it was not fragmented. So when I ran btrfs fi 
defrag syslinux.cfg it did not have any effect and file was not 
uncompressed. Then I simply re-saved the file and it got uncompressed 
fine since bootloader could read it on reboot.

thanks