Re: [PATCH 5/5] btrfs: Show more accurate max_inline

[Nikolay Borisov <nborisov@suse.com>]

On  2.03.2018 10:21, Misono, Tomohiro wrote:
> On 2018/03/02 14:22, Qu Wenruo wrote:
>> Btrfs shows max_inline option into kernel message, but for
>> max_inline=4096, btrfs won't really inline 4096 bytes inline data if
>> it's not compressed.
> 
> Hello,
> I have a question.
> 
> man mount(8) says: 
>    max_inline=bytes
>           Specify  the  maximum  amount  of space, in bytes, that can be
>           inlined in a metadata B-tree leaf.  The value is specified  in
>           bytes,  optionally with a K, M, or G suffix, case insensitive.
>           In practice, this value is limited by the  root  sector  size,
>           with  some  space  unavailable  due to leaf headers.  For a 4k
>           sectorsize, max inline data is ~3900 bytes.
> 
> So, is the size of 4k-(size of leaf header) actually the maximum value
> of max_inline instead of 4095 for 4k sectorsize?

I think the documentation is wrong. Without patch 3/5 we have the max
inline data size as:

BTRFS_MAX_ITEM_SIZE(info) - BTRFS_FILE_EXTENT_INLINE_DATA_START

so MAX_ITEM_SIZE  = nodesize - sizeof(btrfs_header) -
sizeof(btrfs_item).  So this gives us the data portion in the leaf.

So if we substitute the raw number we get:

16k - 101 - 25 = 16258 bytes.

>From this number we also subtract the offset of disk_bytenr in
btrfs_file_extent_item (which is 21). So we end up with
MAX_INLINE_DATA_SIZE of 16258 - 21 = 16237

With Qu's patch the min_t will always be taking 4095 as the
MAX_INLINE_DATA_SIZE.

> 
> Thanks,
> 
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