Re: [PATCH] btrfs: fix the max chunk size and stripe length calculation

[David Sterba <dsterba@suse.cz>]

On Thu, Aug 18, 2022 at 03:06:44PM +0800, Qu Wenruo wrote:
> [BEHAVIOR CHANGE]
> Since commit f6fca3917b4d ("btrfs: store chunk size in space-info
> struct"), btrfs no longer can create larger data chunks than 1G:
> 
>   mkfs.btrfs -f -m raid1 -d raid0 $dev1 $dev2 $dev3 $dev4
>   mount $dev1 $mnt
> 
>   btrfs balance start --full $mnt
>   btrfs balance start --full $mnt
>   umount $mnt
> 
>   btrfs ins dump-tree -t chunk $dev1 | grep "DATA|RAID0" -C 2
> 
> Before that offending commit, what we got is a 4G data chunk:
> 
> 	item 6 key (FIRST_CHUNK_TREE CHUNK_ITEM 9492758528) itemoff 15491 itemsize 176
> 		length 4294967296 owner 2 stripe_len 65536 type DATA|RAID0
> 		io_align 65536 io_width 65536 sector_size 4096
> 		num_stripes 4 sub_stripes 1
> 
> Now what we got is only 1G data chunk:
> 
> 	item 6 key (FIRST_CHUNK_TREE CHUNK_ITEM 6271533056) itemoff 15491 itemsize 176
> 		length 1073741824 owner 2 stripe_len 65536 type DATA|RAID0
> 		io_align 65536 io_width 65536 sector_size 4096
> 		num_stripes 4 sub_stripes 1
> 
> This will increase the number of data chunks by the number of devices,
> not only increase system chunk usage, but also greatly increase mount
> time.
> 
> Without a properly reason, we should not change the max chunk size.
> 
> [CAUSE]
> Previously, we set max data chunk size to 10G, while max data stripe
> length to 1G.
> 
> Commit f6fca3917b4d ("btrfs: store chunk size in space-info struct")
> completely ignored the 10G limit, but use 1G max stripe limit instead,
> causing above shrink in max data chunk size.
> 
> [FIX]
> Fix the max data chunk size to 10G, and in decide_stripe_size_regular()
> we limit stripe_size to 1G manually.
> 
> This should only affect data chunks, as for metadata chunks we always
> set the max stripe size the same as max chunk size (256M or 1G
> depending on fs size).
> 
> Now the same script result the same old result:
> 
> 	item 6 key (FIRST_CHUNK_TREE CHUNK_ITEM 9492758528) itemoff 15491 itemsize 176
> 		length 4294967296 owner 2 stripe_len 65536 type DATA|RAID0
> 		io_align 65536 io_width 65536 sector_size 4096
> 		num_stripes 4 sub_stripes 1
> 
> Reported-by: Wang Yugui <wangyugui@e16-tech.com>
> Fixes: f6fca3917b4d ("btrfs: store chunk size in space-info struct")
> Signed-off-by: Qu Wenruo <wqu@suse.com>

Added to misc-next, thanks. And thanks to Wang Yugui for the report.