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

[Andrei Borzenkov <arvidjaar@gmail.com>]

On 16.03.2022 21:31, Phillip Susi wrote:
> 
> Andrei Borzenkov <arvidjaar@gmail.com> writes:
> 
>> 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.
> 
> Huh?  You can't really do that because the page cache manages files in
> 4k pages.  If you have a 1M extent and you touch one byte in the file,
> thus making one 4k page dirty, are you really saying that btrfs will
> write that modified 4k page somewhere else and NOT free the 4k block
> that is no longer used?

yes.

>  Why the heck not?
>
Short answer - because it is implemented this way.

There could be arbitrary number of overlapping references to this
extent. To track all of these references on every write to decide
whether extent can be split will likely be prohibitively inefficient.

Alternative is to use fixed sized blocks where freeing space is just a
matter of reference count. This means increasing size of metadata for
keeping track of blocks with unknown impact.