[LSF/MM/BPF TOPIC] : Flexible Data Placement (FDP) availability for kernel space file systems

[Kanchan Joshi]

On 1/18/2024 3:21 AM, Dave Chinner wrote:
> On Wed, Jan 17, 2024 at 12:58:12PM +0100, Javier González wrote:
>> On 16.01.2024 11:39, Viacheslav Dubeyko wrote:
>>>> On Jan 15, 2024, at 8:54 PM, Javier González <javier.gonz at samsung.com> wrote:
>>>>> How FDP technology can improve efficiency and reliability of
>>>>> kernel-space file system?
>>>>
>>>> This is an open problem. Our experience is that making data placement
>>>> decisions on the FS is tricky (beyond the obvious data / medatadata). If
>>>> someone has a good use-case for this, I think it is worth exploring.
>>>> F2FS is a good candidate, but I am not sure FDP is of interest for
>>>> mobile - here ZUFS seems to be the current dominant technology.
>>>>
>>>
>>> If I understand the FDP technology correctly, I can see the benefits for
>>> file systems. :)
>>>
>>> For example, SSDFS is based on segment concept and it has multiple
>>> types of segments (superblock, mapping table, segment bitmap, b-tree
>>> nodes, user data). So, at first, I can use hints to place different segment
>>> types into different reclaim units.
>>
>> Yes. This is what I meant with data / metadata. We have looked also into
>> using 1 RUH for metadata and rest make available to applications. We
>> decided to go with a simple solution to start with and complete as we
>> see users.
> 
> XFS has an abstract type definition for metadata that is uses to
> prioritise cache reclaim (i.e. classifies what metadata is more
> important/hotter) and that could easily be extended to IO hints
> to indicate placement.

That sounds very useful.

> We also have a separate journal IO path, and that is probably the
> hotest LBA region of the filesystem (circular overwrite region)
> which would stand to have it's own classification as well.

In the past, I saw nice wins after separating the journal in XFS and 
Ext4 [1]. This is low-effort, high-gain item.

[1]https://www.usenix.org/system/files/conference/fast18/fast18-rho.pdf