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

[Nikolay Borisov <nborisov@suse.com>]

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)
> 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 ?

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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>>
> 
> 
>