[PATCH] btrfs: uapi: fix stale BTRFS_SYSTEM_CHUNK_ARRAY_SIZE comment

[PATCH] btrfs: uapi: fix stale BTRFS_SYSTEM_CHUNK_ARRAY_SIZE comment

[Sun YangKai]

The comment claims room for "14 chunks with 3 stripes each", which
was correct when the structs were smaller.  Since then three additions
grew each entry:

  commit e17cade25ff8 ("Btrfs: Add chunk uuids and update multi-device back references")
    added btrfs_chunk.length (+8) and stripe.dev_uuid (+16)

  commit 321aecc65671 ("Btrfs: Add RAID10 support")
    added btrfs_chunk.sub_stripes (+2)

A 3-stripe entry now takes:

  sizeof(btrfs_disk_key) + btrfs_chunk_item_size(3)
    = 17 + 80 + 32 * (3 - 1)
    = 161 bytes

2048 / 161 ≈ 12.7, so "14" is no longer achievable.  Update to 12
and add the explicit calculation so it does not rot again.

Signed-off-by: Sun YangKai <sunk67188@gmail.com>
---
 include/uapi/linux/btrfs_tree.h | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/include/uapi/linux/btrfs_tree.h b/include/uapi/linux/btrfs_tree.h
index cc3b9f7dccaf..78404abc4ba2 100644
--- a/include/uapi/linux/btrfs_tree.h
+++ b/include/uapi/linux/btrfs_tree.h
@@ -504,7 +504,11 @@ struct btrfs_header {
 
 /*
  * This is a very generous portion of the super block, giving us room to
- * translate 14 chunks with 3 stripes each.
+ * translate 12 chunks with 3 stripes each.
+ *
+ * Each entry takes sizeof(btrfs_disk_key) + btrfs_chunk_item_size(n),
+ * which for a 3-stripe chunk is 17 + 80 + 32 * (3 - 1) = 161 bytes.
+ * 2048 / 161 ≈ 12.7, so at most 12 entries fit.
  */
 #define BTRFS_SYSTEM_CHUNK_ARRAY_SIZE 2048
 
-- 
2.54.0

Re: [PATCH] btrfs: uapi: fix stale BTRFS_SYSTEM_CHUNK_ARRAY_SIZE comment

[David Sterba]

On Wed, Jun 24, 2026 at 03:08:01PM +0800, Sun YangKai wrote:
> The comment claims room for "14 chunks with 3 stripes each", which
> was correct when the structs were smaller.  Since then three additions
> grew each entry:
> 
>   commit e17cade25ff8 ("Btrfs: Add chunk uuids and update multi-device back references")
>     added btrfs_chunk.length (+8) and stripe.dev_uuid (+16)
> 
>   commit 321aecc65671 ("Btrfs: Add RAID10 support")
>     added btrfs_chunk.sub_stripes (+2)
> 
> A 3-stripe entry now takes:
> 
>   sizeof(btrfs_disk_key) + btrfs_chunk_item_size(3)
>     = 17 + 80 + 32 * (3 - 1)
>     = 161 bytes
> 
> 2048 / 161 ≈ 12.7, so "14" is no longer achievable.  Update to 12
> and add the explicit calculation so it does not rot again.
> 
> Signed-off-by: Sun YangKai <sunk67188@gmail.com>

Reviewed-by: David Sterba <dsterba@suse.com>

> --- a/include/uapi/linux/btrfs_tree.h
> +++ b/include/uapi/linux/btrfs_tree.h
> @@ -504,7 +504,11 @@ struct btrfs_header {
>  
>  /*
>   * This is a very generous portion of the super block, giving us room to
> - * translate 14 chunks with 3 stripes each.
> + * translate 12 chunks with 3 stripes each.
> + *
> + * Each entry takes sizeof(btrfs_disk_key) + btrfs_chunk_item_size(n),
> + * which for a 3-stripe chunk is 17 + 80 + 32 * (3 - 1) = 161 bytes.
> + * 2048 / 161 ≈ 12.7, so at most 12 entries fit.

Nice, thanks.