Btrfs and raw zvol-like partition

Btrfs and raw zvol-like partition

[Sébastien Wacquiez]

Hi,

A nice feature is ZFS is the "ZVOL" layer, that permit you to export=20
(directly) a raw device from your zfs pool of disc, with the benefit of=
=20
powerful (growing!) snapshot and easy raid management from zfs. It's=20
particulary usefull when you use it with virtual server, allowing you t=
o=20
centralize all your backup problematic, etc

Does btrfs plan to support this kind of feature ? (please, don't tell m=
e=20
that lvm do, lvm just sucks when you make a snapshot of your disk, and=20
lack of the "growing", commit, rollback, diffsend feature).

Thanks !


S=E9bastien Wacquiez

PS : see http://opensolaris.org/os/community/zfs/source/zfstour.png if=20
you don't know what zvol do.
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Re: Btrfs and raw zvol-like partition

[Andrey Kuzmin]

zvol (interface) does not just  'export raw device' but rather
implements volume  abstraction and integrates volume management into
file-system.

Regards,
Andrey



On Sun, Apr 12, 2009 at 11:26 AM, S=C3=A9bastien Wacquiez <sw@enix.org>=
 wrote:
> Hi,
>
> A nice feature is ZFS is the "ZVOL" layer, that permit you to export
> (directly) a raw device from your zfs pool of disc, with the benefit =
of
> powerful (growing!) snapshot and easy raid management from zfs. It's
> particulary usefull when you use it with virtual server, allowing you=
 to
> centralize all your backup problematic, etc
>
> Does btrfs plan to support this kind of feature ? (please, don't tell=
 me
> that lvm do, lvm just sucks when you make a snapshot of your disk, an=
d lack
> of the "growing", commit, rollback, diffsend feature).
>
> Thanks !
>
>
> S=C3=A9bastien Wacquiez
>
> PS : see http://opensolaris.org/os/community/zfs/source/zfstour.png i=
f you
> don't know what zvol do.
> --
> 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 =C2=A0http://vger.kernel.org/majordomo-info.ht=
ml
>
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Re: Btrfs and raw zvol-like partition

[Sébastien Wacquiez]

Andrey Kuzmin a =C3=A9crit :
> zvol (interface) does not just  'export raw device' but rather
> implements volume  abstraction and integrates volume management into
> file-system.
>  =20
Yep. I suck at writing english, thanks for pointing that out :)

I surely mislead myself, but I think that the "volume management" of ZF=
S=20
could be done with file.

Alloc on write ? Use sparse file. Resizing ? Append or truncate the=20
file. Snapshot ? Snapshot the file. Another Volume ? An other file :)

In fact, the two thinks that have to be done, for me, is :
 1/ Optimise the different layer to bypass permission, acl, & co, and=20
surely the way data is written.
 2/ Be able to export those file directly as block device. (Allowing=20
some more optimisation :) )

It's why I called this feature "export raw device" (as in "export raw=20
file as block device"). As I say, it could be emulated with file and=20
loopback, but it'll surely be slow ... So I wanna know if btrfs plan to=
=20
have those type of optimization (and the user land tools to simplify=20
it's management).


Regards,


S=C3=A9bastien Wacquiez
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Re: Btrfs and raw zvol-like partition

[Andrey Kuzmin]

On Sun, Apr 12, 2009 at 10:32 PM, S=C3=A9bastien Wacquiez <sw@enix.org>=
 wrote:
> Andrey Kuzmin a =C3=A9crit :
>>
>> zvol (interface) does not just =C2=A0'export raw device' but rather
>> implements volume =C2=A0abstraction and integrates volume management=
 into
>> file-system.
>>
>
> Yep. I suck at writing english, thanks for pointing that out :)
>
> I surely mislead myself, but I think that the "volume management" of =
ZFS
> could be done with file.

Basically volume abstraction aims to provide a layer above raw device
implementing extra functionality not found with raw disk (e.g.,
fault-tolerance), expandability (one can add disks to a volume as
needed) and single point of control over a bunch of disks.

>
> Alloc on write ? Use sparse file. Resizing ? Append or truncate the f=
ile.
> Snapshot ? Snapshot the file. Another Volume ? An other file :)

Basically you're right and, for instance, (deprecated) opensolaris
user-spce iSCSI target supports exporting a regular zfs file to remote
clients as raw scsi block device. At the same time,  implementing a
'volume' on-top of a regular zfs file might be unsafe (file-system
makes some assumptions wrt I/O that are fine with raw devices below
real volume but do not necessarily hold for a regular file) .

>
> In fact, the two thinks that have to be done, for me, is :
> 1/ Optimise the different layer to bypass permission, acl, & co, and =
surely
> the way data is written.
> 2/ Be able to export those file directly as block device. (Allowing s=
ome
> more optimisation :) )
>
> It's why I called this feature "export raw device" (as in "export raw=
 file
> as block device"). As I say, it could be emulated with file and loopb=
ack,
> but it'll surely be slow ... So I wanna know if btrfs plan to have th=
ose
> type of optimization (and the user land tools to simplify it's manage=
ment).
>

That's a question to linux-btrfs@, I'm quite new to this one.

Regards,
Andrey
>
> Regards,
>
>
> S=C3=A9bastien Wacquiez
> --
> To unsubscribe from this list: send the line "unsubscribe linux-btrfs=
" in
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Re: Btrfs and raw zvol-like partition

[Chris Mason]

On Sun, 2009-04-12 at 09:26 +0200, S=C3=A9bastien Wacquiez wrote:
> Hi,
>=20
> A nice feature is ZFS is the "ZVOL" layer, that permit you to export=20
> (directly) a raw device from your zfs pool of disc, with the benefit =
of=20
> powerful (growing!) snapshot and easy raid management from zfs. It's=20
> particulary usefull when you use it with virtual server, allowing you=
 to=20
> centralize all your backup problematic, etc
>=20
> Does btrfs plan to support this kind of feature ? (please, don't tell=
 me=20
> that lvm do, lvm just sucks when you make a snapshot of your disk, an=
d=20
> lack of the "growing", commit, rollback, diffsend feature).
>=20

I've always been interested in a btrfs export via loop or dm.  The
problem with loop is that it needs to be switched over to O_DIRECT or
something similar.

At any rate, it would be a fun project for someone looking to know dm
better.

-chris


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