Re: Fwd: Fwd: Question regarding to Btrfs patchwork /2831525

[Qu Wenruo <quwenruo.btrfs@gmx.com>]

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On 2018年01月14日 18:13, Ilan Schwarts wrote:
> both btrfs filesystems will have same fsid ?
> 
> 
> On Sun, Jan 14, 2018 at 12:06 PM, Ilan Schwarts <ilan84@gmail.com> wrote:
>> But both filesystems will have same fsid?
>>
>> On Jan 14, 2018 12:04, "Nikolay Borisov" <nborisov@suse.com> wrote:
>>>
>>>
>>>
>>> On 14.01.2018 12:02, Ilan Schwarts wrote:
>>>> First of all, Thanks for response !
>>>> So if i have 2 btrfs file system on the same machine (not your
>>>> everyday scenario, i know)

Not a problem, the 2 filesystems will have 2 different fsid.

(And it's my everyday scenario, since fstests neeeds TEST_DEV and
SCRATCH_DEV_POOL)

>>>> Lets say a file is created on device A, the file gets inode number X
>>>> is it possible on device B to have inode number X also ?
>>>> or each device has its own Inode number range ?

Forget the mess about device.

Inode is bounded to a filesystem, not bounded to a device.

Just traditional filesytems are normally bounded to a single device.
(Although even traditional filesystems can have external journal devices)

So there is nothing to do with device at all.

And you can have same inode numbers in different filesystems, but
BTRFS_I(inode)->root->fs_info will point to different fs_infos, with
different fsid.

So return to your initial question:
> both btrfs filesystems will have same fsid ?

No, different filesystems will have different fsid.

(Unless you're SUUUUUUUUUUUUUUUUUUUPER lucky to have 2 filesystems with
same fsid)

Thanks,
Qu


>>>
>>> Of course it is possible. Inodes are guaranteed to be unique only across
>>> filesystem instances. In your case you are going to have 2 fs instances.
>>>
>>>>
>>>> I need to create unique identifier for a file, I need to understand if
>>>> the identifier would be: GlobalFSID_DeviceID_Inode or DeviceID_Inode
>>>> is enough.
>>>>
>>>> Thanks
>>>>
>>>>
>>>>
>>>>
>>>>
>>>> On Sun, Jan 14, 2018 at 11:13 AM, Qu Wenruo <quwenruo.btrfs@gmx.com>
>>>> wrote:
>>>>>
>>>>>
>>>>> On 2018年01月14日 16:33, Ilan Schwarts wrote:
>>>>>> Hello btrfs developers/users,
>>>>>>
>>>>>> I was wondering regarding to fetching the correct fsid on btrfs from
>>>>>> the context of a kernel module.
>>>>>
>>>>> There are two IDs for btrfs. (in fact more, but you properly won't need
>>>>> the extra ids)
>>>>>
>>>>> FSID: Global one, one fs one FSID.
>>>>> Device ID: Bonded to device, each device will have one.
>>>>>
>>>>> So in case of 2 devices btrfs, each device will has its own device id,
>>>>> while both of the devices have the same fsid.
>>>>>
>>>>> And I think you're talking about the global fsid instead of device id.
>>>>>
>>>>>> if on suse11.3 kernel 3.0.101-0.47.71-default in order to get fsid, I
>>>>>> do the following:
>>>>>> convert inode struct to btrfs_inode struct (use btrfsInode =
>>>>>> BTRFS_I(inode)), then from btrfs_inode struct i go to root field, and
>>>>>> from root i take anon_dev or anon_super.s_dev.
>>>>>>         struct btrfs_inode *btrfsInode;
>>>>>>         btrfsInode = BTRFS_I(inode);
>>>>>>        btrfsInode->root->anon_super.s_dev    or
>>>>>>        btrfsInode->root->anon_dev    - depend on kernel.
>>>>>
>>>>> The most directly method would be:
>>>>>
>>>>> btrfs_inode->root->fs_info->fsid.
>>>>> (For newer kernel, as I'm not familiar with older kernels)
>>>>>
>>>>> Or from superblock:
>>>>> btrfs_inode->root->fs_info->super_copy->fsid.
>>>>> (The most reliable one, no matter which kernel version you're using, as
>>>>> long as the super block format didn't change)
>>>>>
>>>>> For device id, it's not that commonly used unless you're dealing with
>>>>> chunk mapping, so I'm assuming you're referring to fsid.
>>>>>
>>>>> Thanks,
>>>>> Qu
>>>>>
>>>>>>
>>>>>> In kernel 3.12.28-4-default in order to get the fsid, i need to go
>>>>>> to the inode -> superblock -> device id (inode->i_sb->s_dev)
>>>>>>
>>>>>> Why is this ? and is there a proper/an official way to get it ?
>>>>>> --
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>>>>>> in
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>>>>>>
>>>>>
>>>>
>>>>
>>>>
> 
> 
> 


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