On 11/5/2018 11:53 AM, valdis.kletnieks@xxxxxx wrote:
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...
Indeed, and I can't really comment on how it might behave under those
circumstances (the systems I did the testing on never saw memory
pressure quite _that_ bad, and I had them set up to swap things out
pretty early and really aggressively).
