Re: Btrfs autodefrag wrote 5TB in one day to a 0.5TB SSD without a measurable benefit

[Andrei Borzenkov <arvidjaar@gmail.com>]

On 14.03.2022 23:02, Phillip Susi wrote:
> 
> Jan Ziak <0xe2.0x9a.0x9b@gmail.com> writes:
> 
>> The actual disk usage of a file in a copy-on-write filesystem can be
>> much larger than sb.st_size obtained via fstat(fd, &sb) if, for
>> example, a program performs many (millions) single-byte file changes
>> using write(fd, buf, 1) to distinct/random offsets in the file.
> 
> How?  I mean if you write to part of the file a new block is written
> somewhere else but the original one is then freed, so the overall size
> should not change.  Just because all of the blocks of the file are not
> contiguous does not mean that the file has more of them, and making them
> contiguous does not reduce the number of them.
> 

btrfs does not manage space in fixed size blocks. You describe behavior
of WAFL.

btrfs manages space in variable size extents. If you change 999 bytes in
1000 bytes extent, original extent remains allocated because 1 byte is
still referenced. So actual space consumption is now 1999 bytes.