ZFS with large blocks on 10.2-PRERELEASE r286065: still have big problems with [kmem arena]

[Lev Serebryakov]

Hello Freebsd-fs,

 I've rebuilt OS to ~latest version to have VM-related fixes, and still have
problems with processes stuck in "kmem arena" state when they access files
on ZFS with large blocks.

 I have ARC limited to 3GiB (out of 6GiB of physical memory), and ZFS with
large files (1Gb+) and large blocks (16MiB) on radiz vdev (5 disks). Simple
"dd if=large.file of=/dev/null bs=1m" could spent in "kmem arena" state

 Looks like such large blocks was bad idea, but anyway 500Kb/s linear reading?
Really?

 What is strange, CPU is totally idle when reading process is stuck. No
kernel threads are working for tens of minutes!

 I'm rebuilding pool with more resonable block sizes (and lets it waste
space and time for metadata), but this situation (no work could be done when
CPU is totally idle) looks wired and ugly anyway.

-- 
Best regards,
 Lev                          mailto:lev at FreeBSD.org