Re: How about adding an ioctl to convert a directory to a subvolume?

[Lu Fengqi <lufq.fnst@cn.fujitsu.com>]

On Tue, Nov 28, 2017 at 07:48:28PM +0100, David Sterba wrote:
>On Mon, Nov 27, 2017 at 05:41:56PM +0800, Lu Fengqi wrote:
>> As we all know, under certain circumstances, it is more appropriate to
>> create some subvolumes rather than keep everything in the same
>> subvolume. As the condition of demand change, the user may need to
>> convert a previous directory to a subvolume. For this reason，how about
>> adding an ioctl to convert a directory to a subvolume?

Thanks for taking time to reply.

>
>I'd say too difficult to get everything right in kernel. This is
>possible to be done in userspace, with existing tools.
>
>The problem is that the conversion cannot be done atomically in most
>cases, so even if it's just one ioctl call, there are several possible
>intermediate states that would exist during the call. Reporting where
>did the ioctl fail would need some extended error code semantics.

Make sense. 

>
>> Users can convert by the scripts mentioned in this
>> thread(https://www.spinics.net/lists/linux-btrfs/msg33252.html), but is
>> it easier to use the off-the-shelf btrfs subcommand?
>
>Adding a subcommand would work, though I'd rather avoid reimplementing
>'cp -ax' or 'rsync -ax'.  We want to copy the files preserving all
>attributes, with reflink, and be able to identify partially synced
>files, and not cross the mountpoints or subvolumes.

I agree that re-implementation of cp is worthless. 'cp -ax
--reflink=always' already meet the above requirements, except identifing
partially synced files.

In fact, I'm not sure what the partially synced files mean? The files
being partially written to disk? If yes, why we need to identify these?
Or this refers to the open and live files mentioned below?

>
>The middle step with snapshotting the containing subvolume before
>syncing the data is also a valid option, but not always necessary.
>
>> After an initial consideration, our implementation is broadly divided
>> into the following steps:
>> 1. Freeze the filesystem or set the subvolume above the source directory
>> to read-only;
>
>Freezing the filesystme will freeze all IO, so this would not work, but
>I understand what you mean. The file data are synced before the snapshot
>is taken, but nothing prevents applications to continue writing data.
>
>Open and live files is a problem and don't see a nice solution here.
>
>> 2. Perform a pre-check, for example, check if a cross-device link
>> creation during the conversion;
>
>Cross-device links are not a problem as long as we use 'cp' ie. the
>manual creation of files in the target.

└── subvol_1
    ├── dir_1
    │   └── file_1_hard_link
    └── file_1

If we want to convert dir_1 to a new subvolume, we can't create the hard
link of file1 in the new subvolume so that we must abort the conversion.

>
>> 3. Perform conversion, such as creating a new subvolume and moving the
>> contents of the source directory;
>> 4. Thaw the filesystem or restore the subvolume writable property.
>> 
>> In fact, I am not so sure whether this use of freeze is appropriate
>> because the source directory the user needs to convert may be located
>> at / or /home and this pre-check and conversion process may take a long
>> time, which can lead to some shell and graphical application suspended.
>
>I think the closest operation is a read-only remount, which is not
>always possible due to open files and can otherwise considered as quite
>intrusive operation to the whole system. And the root filesystem cannot
>be easily remounted read-only in the systemd days anyway.
>
>

-- 
Thanks,
Lu