Most of io workers in btrfs don''t take into account amount of disks they deal with: fs/btrfs/disk-io.c: fs_info->thread_pool_size = min_t(unsigned long, num_online_cpus() + 2, 8); It might not be a problem for ''write-only'' workloads, but it''s a serious problem for read/write ones. Let''s consider simple setup: - dualcore (core2) laptop with sole HDD (5200 rpm) - slightly aged btrfs: ~50% of 200GB is filled and used as / (nothing special: stock mkfs.btrfs /dev/root) - kernel does not matter much. any 3.3.0+ would fit. 3.5.0-rc3 runs here - tried both anticipatory and deadline schedulers. does not seem to matter that much My "benchmark"[1] is gcc unpacking/perm-sanitizing. It emulates typical workload of source based package manager. When [1] is ran as ''sh torture.sh'': - first we see at least 4 ''btrfs-endio-wri'' hammering disk with random reads and writes in the ''untar'' phase - then we see 4 ''btrfs-delayed-m'' with similar effect in the ''chmod'' phase If it would be a quad-cpu laptop (typical i5) we would get 6(!) threads chewing disk, so you get worse behaviour. System becomes completely unusable. Right now I try to mitigate it with ''thread_pool'' mount option, but it looks like a crude hack. Would be nice to have amount of parallel readers comparable to number underlying spinning devices. Thanks for your patience! [1] torture.sh: #!/bin/sh gcc_url=distfiles.gentoo.org/distfiles/gcc-4.7.0.tar.bz2 gcc_tarball=gcc-4.7.0.tar.bz2 gcc_dir=gcc-4.7.0 [ -f "$gcc_tarball" ] || wget "$gcc_url" chatty() { echo "RUN: $@" /usr/bin/time "$@" } torture() { chatty rm -rf "${gcc_dir}" ### iotop pattern: scattered seeks/reads to death #25499 be/4 root 60.34 K/s 79.20 K/s 0.00 % 99.99 % [btrfs-endio-wri] #28151 be/4 root 56.57 K/s 60.34 K/s 0.00 % 99.73 % [btrfs-endio-wri] # 6181 be/4 root 56.57 K/s 71.66 K/s 0.00 % 96.96 % [flush-btrfs-1] #23881 be/4 root 52.80 K/s 52.80 K/s 0.00 % 93.70 % [btrfs-endio-wri] chatty tar -xjf "${gcc_tarball}" ### iotop pattern: scattered seeks/reads to death #29109 be/4 slyfox 870.05 K/s 0.00 B/s 0.00 % 97.05 % chmod -R 700 gcc-4.7.0/ #28067 be/4 root 162.66 K/s 949.49 K/s 0.00 % 77.72 % [btrfs-delayed-m] #28164 be/4 root 22.70 K/s 215.62 K/s 0.00 % 14.23 % [btrfs-delayed-m] # 4690 be/4 root 0.00 B/s 15.13 K/s 0.00 % 0.00 % [btrfs-delayed-m] echo "now look at iotop" chatty chmod -R 700 "${gcc_dir}"/ chatty sync } torture torture torture torture -- Sergei