Re: [RFE] xfs_growfs: option to clamp growth to an AG boundary

["Darrick J. Wong" <djwong@kernel.org>]

On Mon, Feb 23, 2026 at 02:48:48PM +0500, Марк Коренберг wrote:
> Hi,
> 
> I ran into an issue after growing an XFS filesystem where the final
> allocation group (last AG) ended up very small. Most workloads were
> fine, but large reflink-heavy copies started failing. In my case,
> copying a ClickHouse data directory with:
> 
> `cp -a --reflink=always ...`
> 
> fails on a filesystem with a tiny last AG. Using --reflink=auto

How does it fail?

--D

> doesn’t help either, because `cp` doesn’t fall back to a non-reflink
> copy if the reflink attempt fails.
> 
> To work around this, I had to write scripts that compute a “safe”
> target size before running xfs_growfs. The alignment I needed is a bit
> awkward:
> 
> 1. Round the LV size up to the next multiple of the filesystem AG
> size, so the grown filesystem ends exactly on an AG boundary (no
> partial/tiny last AG).
> 
> 2. Then round the LV size down to the LVM extents size (4 MiB in my
> case). Rounding up to the LVM granularity can reintroduce a tiny last
> AG.
> If the automatically chosen AG size were aligned to that granularity,
> step (2) wouldn’t be necessary.
> 
> This feels like something xfsprogs could support directly. My proposals:
> 
> 1. xfs_growfs: add an option to print an “optimal grow target size”:
> the current(new) block device size rounded **down** to a multiple of
> the AG size.
> A --json output mode would make this easy to consume from scripts.
> 
> 2. AG size calculation/alignment: when choosing an automatic AG size,
> always round it down to an alignment such as 4 MiB, or (preferably)
> consider the underlying device/LVM extent size when it can be
> detected, instead of using a constant.
> 
> 3. Docs (mkfs + AG sizing): when specifying AG size manually,
> recommend: choosing filesystem sizing so the final size is an integer
> multiple of AG size (i.e., no partial last AG), and aligning the AG
> size to the underlying allocation granularity (e.g., LVM
> extent/segment size) when applicable.
> 
> 4. Docs (xfs_growfs): add a note that it’s highly preferable to grow
> the filesystem in multiples of the existing AG size, to avoid a tiny
> last AG.
> 
> 5. Optional grow mode: add a xfs_growfs mode/switch that grows “as
> much as possible”, but clamps the resulting filesystem size **down**
> to an AG boundary, and reports how much space is left unused (e.g., “X
> bytes left unallocated to avoid a partial final AG”).
> 
> This might sound like a corner case, but it’s easy to hit in practice
> when the block device is resized to just arbitrary chosen size then
> xfs_growfs expands to consume the whole device.
> 
> Thanks,
> Mark
>