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

[Qu Wenruo <quwenruo.btrfs@gmx.com>]

On 2022/8/18 16:04, Wang Yugui wrote:
> Hi,
>
>> [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>
>> ---
>>   fs/btrfs/space-info.c | 2 +-
>>   fs/btrfs/volumes.c    | 3 +++
>>   2 files changed, 4 insertions(+), 1 deletion(-)
>>
>> diff --git a/fs/btrfs/space-info.c b/fs/btrfs/space-info.c
>> index 477e57ace48d..b74bc31e9a8e 100644
>> --- a/fs/btrfs/space-info.c
>> +++ b/fs/btrfs/space-info.c
>> @@ -199,7 +199,7 @@ static u64 calc_chunk_size(const struct btrfs_fs_info *fs_info, u64 flags)
>>   	ASSERT(flags & BTRFS_BLOCK_GROUP_TYPE_MASK);
>>
>>   	if (flags & BTRFS_BLOCK_GROUP_DATA)
>> -		return SZ_1G;
>> +		return BTRFS_MAX_DATA_CHUNK_SIZE;
>>   	else if (flags & BTRFS_BLOCK_GROUP_SYSTEM)
>>   		return SZ_32M;
>>
>> diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
>> index 8c64dda69404..e0fd1aecf447 100644
>> --- a/fs/btrfs/volumes.c
>> +++ b/fs/btrfs/volumes.c
>> @@ -5264,6 +5264,9 @@ static int decide_stripe_size_regular(struct alloc_chunk_ctl *ctl,
>>   				       ctl->stripe_size);
>>   	}
>>
>> +	/* Stripe size should never go beyond 1G. */
>
> Currently we  limit  the stripe size to SIZE_1G.
>
> Is there some technical limit such as 'used as signed 32bit, so max
> SIZE_1G or SIZE_2G?' or 'used as unsigned 32bit, so max SIZE_2G or
> SIZE_4G?'

AFAIK the only problem with larger stripe size is for unbalanced
data/metadata case it will be harder to reclaim block groups.

Other than that I'm not aware of certain problems related the stripe size.

Thanks,
Qu
>
> Best Regards
> Wang Yugui (wangyugui@e16-tech.com)
> 2022/08/18
>
>
>> +	ctl->stripe_size = min_t(u64, ctl->stripe_size, SZ_1G);
>> +
>>   	/* Align to BTRFS_STRIPE_LEN */
>>   	ctl->stripe_size = round_down(ctl->stripe_size, BTRFS_STRIPE_LEN);
>>   	ctl->chunk_size = ctl->stripe_size * data_stripes;
>> --
>> 2.37.1
>
>