Re: [PATCH 11/24] xfs: create incore realtime group structures

Darrick J. Wong
2024-08-27T05:17:19Z

On Mon, Aug 26, 2024 at 09:38:16PM -0700, Christoph Hellwig wrote:
> On Tue, Aug 27, 2024 at 10:57:34AM +1000, Dave Chinner wrote:
> > We're discussing how to use the sparse fsbno addressing to allow
> > resizing of AGs, but we will not be able to do that at all with
> > rtgroups as they stand. The limitation is a 64 bit global rt extent
> > address is essential the physical address of the extent in the block
> > device LBA space.
>
> With this series there are not global RT extent addresses, the extents
> are always relative to the group and an entity only used in the
> allocator.
>
> > /*
> > * xfs_group - a contiguous 32 bit block address space group
> > */
> > struct xfs_group {
> > struct xarray xarr;
> > u32 num_groups;
> > };
> >
> > struct xfs_group_item {
> > struct xfs_group *group; /* so put/rele don't need any other context */
> > u32 gno;
> > atomic_t passive_refs;
> > atomic_t active_refs;
>
> What is the point of splitting the group and group_item? This isn't
> done in the current perag struture either.

I think xfs_group encapsulates/replaces the radix tree root in struct
xfs_mount, and the xarray inside it points to xfs_group_item objects.

> > Hence I'm wondering if we should actually cap the maximum number of
> > rtgroups. WE're just about at BS > PS, so with a 64k block size a
> > single rtgroup can index 2^32 * 2^16 bytes which puts individual
> > rtgs at 256TB in size. Unless there are use cases for rtgroup sizes
> > smaller than a few GBs, I just don't see the need for support
> > theoretical maximum counts on tiny block size filesystems. Thirty
> > thousand rtgs at 256TB per rtg puts us at 64 bit device size limits,
> > and we hit those limits on 4kB block sizes at around 500,000 rtgs.
> >
> > So do we need to support millions of rtgs? I'd say no....
>
> As said before hardware is having a word with with the 256GB hardware
> zone size in SMR HDDs. I hope that size will eventually increase, but
> I would not bet my house on it.

Wait, 256 *gigabytes*? That wouldn't be such a bad minimum.

--D