On Mon, 05 Nov 2018 11:28:49 -0500, "Austin S. Hemmelgarn" said:

> Also, it's probably worth noting that BTRFS doesn't need to decompress
> the entire file to read or write blocks in the middle, it splits the
> file into 128k blocks and compresses each of those independent of the
> others, so it can just decompress the 128k block that holds the actual
> block that's needed.

Presumably it does something sane with block allocation for the now-compressed
128K that's presumably much smaller.  Also, that limits the damage from writing to
the middle of a compression unit....

That *does* however increase the memory requirement - you can OOM or
deadlock if your read/write from the swap needs an additional 128K for the
compression buffer at an inconvenient time...