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List: linux-btrfs
Subject: R: Re: [PATCH 5/5] btrfs: Add ioctl to set snapshot
From: "Goffredo Baroncelli <kreijack () libero ! it>" <kreijack () libero ! it>
Date: 2010-11-30 7:52:53
Message-ID: 13025594.9851291103573101.JavaMail.defaultUser () defaultHost
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Hi Li,
>----Messaggio originale----
>Da: lizf@cn.fujitsu.com
>Data: 30/11/2010 8.03
>A: <kreijack@libero.it>
>Cc: <linux-btrfs@vger.kernel.org>
>Ogg: Re: [PATCH 5/5] btrfs: Add ioctl to set snapshot readonly/writable
>
>Goffredo Baroncelli wrote:
>> Hi Li,
>>
>> On Monday, 29 November, 2010, Li Zefan wrote:
>>> This allows us to set a snapshot readonly or writable on the fly.
>>>
>>> Usage:
>>>
>>> Set BTRFS_SNAPSHOT_RDONLY/WRITABLE of btrfs_ioctl_vol_arg_v2->flags,
>>> and then call ioctl(BTRFS_IOCTL_SNAP_SETFLAGS);
>>
>> I really appreciate your work, but I have some doubt about this interface.
In
>> particolar:
>
>It's the interface that I would like to be discussed. Thanks!
>
>> - how get the flags of a subvolume ? I suggest to implement a pair of
ioctls:
>> - subvolume_setflags -> get the flags
>> - subvolume_getflags -> set the flags
>> These ioctls would be more generic (there are a lot of flags which may be
>> interested to put in the "root" of a subvolume: think about
>> compress/nocompress, (no)datasum...)
>> - For the reason abowe, I suggest to replace SNAPSHOT with SUBVOLUME
>> - Finally, with a pair of get/set_flags functions we can avoid the use of
the
>> flags BTRFS_SNAPSHOT_WRITABLE.
>>
>
>There are some reasons that I created this interface:
>
>- set/getflags should set/get root flags which reflect in struct
>btrfs_root_item->flags.
>
>- btrfs_root_item->flags was not used at all before this patch, so
>(no)compress and (no)datasum is not reflect in ->flags.
>
>- _CREATE_ASYNC flag is to create snapshot asynchronously, so it's not
>a flag of tree root.
Of course I never mind about _CREATE_ASYNC to be set in btrfs_root_item-
>flags.
_CREATE_ASYNC is not a snapshot properties but a way of creating a subvolume.
But other flags may make sense to live in btrfs_root_item->flags.
So I am suggesting to develop a more general
interface for future improvement. These pair of functions (*_set/get) should
be use to
set the subvolume/snapshot properties. And the RDONLY is one of them.
In the detail to set an attributue an user should be:
- get the subvolume flags (ioctl(fd, BTRFS_IOC_SNAP_GETFLAGS....) )
- compute the new flags ( flags |= BTRFS_FLAGS_XXXX or flags &=
~BTRFS_FLAG_XXXX)
- set the subvolume flags (ioctl(fd, BTRFS_IOC_SNAP_SGETFLAGS....) )
>
>- It seems to me there's no user requirement for getflags ioctl to
>return _RDONLY/_WRITABLE flags of a tree root?
And how an user/admin can understand that a snapshot/subvolume is readonly ?
How
>
>- By suggesting BTRFS_SUBVOL_RDONLY, does it impliy not only snapshot
>but also a subvolume can be made readonly?
IIRC a snapshot is a subvolume already filled from the beginning. Why doesn't
share
the capability of make a subvolume RO ?
Finally I have another suggestion: make sense to check that the file
descriptor is referring
to the root of a subvolume instead of a the tree. I highlight that because the
other ioctls
suffer the same problem and confused the user sometime. For example a lot of
people tough that was possible to snapshot a directory, because the ioctl
doesn't return
any error. But instead of the directory the snapshot was of the full
subvolume.
Goffredo
[...]
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