I/O for multiple programs, like `thunderbird`, `firefox`, etc., hangs for minutes, `dmesg` contains

    [ 9184.451606] INFO: task txg_sync:11471 blocked for more than 120 seconds.
    [ 9184.451610]       Tainted: P           OE   4.8.0-32-generic #34-Ubuntu
    [ 9184.451611] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
    [ 9184.451612] txg_sync        D ffffa240ab3a7aa8     0 11471      2 0x00000000
    [ 9184.451616]  ffffa240ab3a7aa8 00ffffffbb6ade1f ffffa24095148000 ffffa240e5ca5580
    [ 9184.451618]  0000000000000046 ffffa240ab3a8000 ffffa240ff359200 7fffffffffffffff
    [ 9184.451620]  ffffa23d36cf9050 0000000000000001 ffffa240ab3a7ac0 ffffffffbbe96b15
    [ 9184.451621] Call Trace:
    [ 9184.451627]  [<ffffffffbbe96b15>] schedule+0x35/0x80
    [ 9184.451628]  [<ffffffffbbe9a2ca>] schedule_timeout+0x22a/0x3f0
    [ 9184.451631]  [<ffffffffbb62c77e>] ? __switch_to+0x2ce/0x6c0
    [ 9184.451633]  [<ffffffffbb6c156c>] ? pick_next_task_fair+0x48c/0x4c0
    [ 9184.451635]  [<ffffffffbb6fc511>] ? ktime_get+0x41/0xb0
    [ 9184.451636]  [<ffffffffbbe96304>] io_schedule_timeout+0xa4/0x110
    [ 9184.451644]  [<ffffffffc0acee12>] cv_wait_common+0xb2/0x130 [spl]
    [ 9184.451646]  [<ffffffffbb6c7150>] ? wake_atomic_t_function+0x60/0x60
    [ 9184.451650]  [<ffffffffc0aceee8>] __cv_wait_io+0x18/0x20 [spl]
    [ 9184.451689]  [<ffffffffc0ebfcdd>] zio_wait+0xfd/0x1d0 [zfs]
    [ 9184.451716]  [<ffffffffc0e4b2c8>] dsl_pool_sync+0xb8/0x480 [zfs]
    [ 9184.451745]  [<ffffffffc0e6650f>] spa_sync+0x37f/0xb30 [zfs]
    [ 9184.451747]  [<ffffffffbb6aecd2>] ? default_wake_function+0x12/0x20
    [ 9184.451779]  [<ffffffffc0e77195>] txg_sync_thread+0x3a5/0x600 [zfs]
    [ 9184.451807]  [<ffffffffc0e76df0>] ? txg_delay+0x160/0x160 [zfs]
    [ 9184.451811]  [<ffffffffc0ac9fb1>] thread_generic_wrapper+0x71/0x80 [spl]
    [ 9184.451815]  [<ffffffffc0ac9f40>] ? __thread_exit+0x20/0x20 [spl]
    [ 9184.451817]  [<ffffffffbb6a3e58>] kthread+0xd8/0xf0
    [ 9184.451819]  [<ffffffffbbe9b4df>] ret_from_fork+0x1f/0x40
    [ 9184.451821]  [<ffffffffbb6a3d80>] ? kthread_create_on_node+0x1e0/0x1e0
    [ 9184.451849] INFO: task mozStorage #2:21607 blocked for more than 120 seconds.
    [ 9184.451851]       Tainted: P           OE   4.8.0-32-generic #34-Ubuntu
    [ 9184.451852] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
    [ 9184.451853] mozStorage #2   D ffffa23fe8a5bd38     0 21607  19750 0x00000004
    [ 9184.451855]  ffffa23fe8a5bd38 00ffa240ee8feb40 ffffa240ecf72ac0 ffffa2403803b900
    [ 9184.451857]  ffffffffbc2c02f7 ffffa23fe8a5c000 ffffa240aa940828 ffffa240aa940800
    [ 9184.451858]  ffffa240aa940980 0000000000000000 ffffa23fe8a5bd50 ffffffffbbe96b15
    [ 9184.451860] Call Trace:
    [ 9184.451861]  [<ffffffffbbe96b15>] schedule+0x35/0x80
    [ 9184.451866]  [<ffffffffc0acee70>] cv_wait_common+0x110/0x130 [spl]
    [ 9184.451868]  [<ffffffffbb6c7150>] ? wake_atomic_t_function+0x60/0x60
    [ 9184.451872]  [<ffffffffc0aceea5>] __cv_wait+0x15/0x20 [spl]
    [ 9184.451904]  [<ffffffffc0eb9759>] zil_commit.part.11+0x79/0x7a0 [zfs]
    [ 9184.451909]  [<ffffffffc0acf9f6>] ? tsd_hash_search.isra.0+0x46/0xa0 [spl]
    [ 9184.451913]  [<ffffffffc0ad0214>] ? tsd_set+0x2b4/0x500 [spl]
    [ 9184.451914]  [<ffffffffbbe98a82>] ? mutex_lock+0x12/0x30
    [ 9184.451945]  [<ffffffffc0eb9e97>] zil_commit+0x17/0x20 [zfs]
    [ 9184.451975]  [<ffffffffc0eaf2aa>] zfs_fsync+0x7a/0xf0 [zfs]
    [ 9184.452005]  [<ffffffffc0ec5878>] zpl_fsync+0x68/0xa0 [zfs]
    [ 9184.452008]  [<ffffffffbb868b9b>] vfs_fsync_range+0x4b/0xb0
    [ 9184.452010]  [<ffffffffbb868c5d>] do_fsync+0x3d/0x70
    [ 9184.452011]  [<ffffffffbb868ef0>] SyS_fsync+0x10/0x20
    [ 9184.452013]  [<ffffffffbbe9b2b6>] entry_SYSCALL_64_fastpath+0x1e/0xa8
    [ 9184.452023] INFO: task bitcoin-msghand:663 blocked for more than 120 seconds.
    [ 9184.452024]       Tainted: P           OE   4.8.0-32-generic #34-Ubuntu
    [ 9184.452025] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
    [ 9184.452026] bitcoin-msghand D ffffa23eeb23bd38     0   663  26994 0x00000000
    [ 9184.452028]  ffffa23eeb23bd38 00ffa23eab434000 ffffa240ecf70000 ffffa24095148000
    [ 9184.452030]  ffffa23eeb23bd20 ffffa23eeb23c000 ffffa240aa940828 ffffa240aa940800
    [ 9184.452031]  ffffa240aa940980 0000000000000000 ffffa23eeb23bd50 ffffffffbbe96b15
    [ 9184.452033] Call Trace:
    [ 9184.452034]  [<ffffffffbbe96b15>] schedule+0x35/0x80
    [ 9184.452039]  [<ffffffffc0acee70>] cv_wait_common+0x110/0x130 [spl]
    [ 9184.452041]  [<ffffffffbb6c7150>] ? wake_atomic_t_function+0x60/0x60
    [ 9184.452044]  [<ffffffffc0aceea5>] __cv_wait+0x15/0x20 [spl]
    [ 9184.452074]  [<ffffffffc0eb9759>] zil_commit.part.11+0x79/0x7a0 [zfs]
    [ 9184.452079]  [<ffffffffc0acf9f6>] ? tsd_hash_search.isra.0+0x46/0xa0 [spl]
    [ 9184.452083]  [<ffffffffc0ad0214>] ? tsd_set+0x2b4/0x500 [spl]
    [ 9184.452084]  [<ffffffffbbe98a82>] ? mutex_lock+0x12/0x30
    [ 9184.452113]  [<ffffffffc0eb9e97>] zil_commit+0x17/0x20 [zfs]
    [ 9184.452141]  [<ffffffffc0eaf2aa>] zfs_fsync+0x7a/0xf0 [zfs]
    [ 9184.452168]  [<ffffffffc0ec5878>] zpl_fsync+0x68/0xa0 [zfs]
    [ 9184.452170]  [<ffffffffbb868b9b>] vfs_fsync_range+0x4b/0xb0
    [ 9184.452172]  [<ffffffffbb868c5d>] do_fsync+0x3d/0x70
    [ 9184.452173]  [<ffffffffbb868f13>] SyS_fdatasync+0x13/0x20
    [ 9184.452174]  [<ffffffffbbe9b2b6>] entry_SYSCALL_64_fastpath+0x1e/0xa8
    [ 9305.221219] INFO: task txg_sync:11471 blocked for more than 120 seconds.
    [ 9305.221223]       Tainted: P           OE   4.8.0-32-generic #34-Ubuntu
    [ 9305.221224] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
    [ 9305.221225] txg_sync        D ffffa240ab3a7aa8     0 11471      2 0x00000000
    [ 9305.221229]  ffffa240ab3a7aa8 00ffffffbb6ade1f ffffa24095148000 ffffa240e5ca5580
    [ 9305.221230]  0000000000000046 ffffa240ab3a8000 ffffa240ff359200 7fffffffffffffff
    [ 9305.221232]  ffffa23d36cf9050 0000000000000001 ffffa240ab3a7ac0 ffffffffbbe96b15
    [ 9305.221234] Call Trace:
    [ 9305.221239]  [<ffffffffbbe96b15>] schedule+0x35/0x80
    [ 9305.221241]  [<ffffffffbbe9a2ca>] schedule_timeout+0x22a/0x3f0
    [ 9305.221243]  [<ffffffffbb62c77e>] ? __switch_to+0x2ce/0x6c0
    [ 9305.221245]  [<ffffffffbb6c156c>] ? pick_next_task_fair+0x48c/0x4c0
    [ 9305.221247]  [<ffffffffbb6fc511>] ? ktime_get+0x41/0xb0
    [ 9305.221249]  [<ffffffffbbe96304>] io_schedule_timeout+0xa4/0x110
    [ 9305.221256]  [<ffffffffc0acee12>] cv_wait_common+0xb2/0x130 [spl]
    [ 9305.221258]  [<ffffffffbb6c7150>] ? wake_atomic_t_function+0x60/0x60
    [ 9305.221262]  [<ffffffffc0aceee8>] __cv_wait_io+0x18/0x20 [spl]
    [ 9305.221301]  [<ffffffffc0ebfcdd>] zio_wait+0xfd/0x1d0 [zfs]
    [ 9305.221328]  [<ffffffffc0e4b2c8>] dsl_pool_sync+0xb8/0x480 [zfs]
    [ 9305.221357]  [<ffffffffc0e6650f>] spa_sync+0x37f/0xb30 [zfs]
    [ 9305.221359]  [<ffffffffbb6aecd2>] ? default_wake_function+0x12/0x20
    [ 9305.221389]  [<ffffffffc0e77195>] txg_sync_thread+0x3a5/0x600 [zfs]
    [ 9305.221417]  [<ffffffffc0e76df0>] ? txg_delay+0x160/0x160 [zfs]
    [ 9305.221421]  [<ffffffffc0ac9fb1>] thread_generic_wrapper+0x71/0x80 [spl]
    [ 9305.221424]  [<ffffffffc0ac9f40>] ? __thread_exit+0x20/0x20 [spl]
    [ 9305.221426]  [<ffffffffbb6a3e58>] kthread+0xd8/0xf0
    [ 9305.221428]  [<ffffffffbbe9b4df>] ret_from_fork+0x1f/0x40
    [ 9305.221430]  [<ffffffffbb6a3d80>] ? kthread_create_on_node+0x1e0/0x1e0
    [ 9305.221458] INFO: task mozStorage #2:21607 blocked for more than 120 seconds.
    [ 9305.221459]       Tainted: P           OE   4.8.0-32-generic #34-Ubuntu
    [ 9305.221460] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
    [ 9305.221461] mozStorage #2   D ffffa23fe8a5bd38     0 21607  19750 0x00000004
    [ 9305.221463]  ffffa23fe8a5bd38 00ffa240ee8feb40 ffffa240ecf72ac0 ffffa2403803b900
    [ 9305.221465]  ffffffffbc2c02f7 ffffa23fe8a5c000 ffffa240aa940828 ffffa240aa940800
    [ 9305.221466]  ffffa240aa940980 0000000000000000 ffffa23fe8a5bd50 ffffffffbbe96b15
    [ 9305.221468] Call Trace:
    [ 9305.221469]  [<ffffffffbbe96b15>] schedule+0x35/0x80
    [ 9305.221474]  [<ffffffffc0acee70>] cv_wait_common+0x110/0x130 [spl]
    [ 9305.221475]  [<ffffffffbb6c7150>] ? wake_atomic_t_function+0x60/0x60
    [ 9305.221479]  [<ffffffffc0aceea5>] __cv_wait+0x15/0x20 [spl]
    [ 9305.221510]  [<ffffffffc0eb9759>] zil_commit.part.11+0x79/0x7a0 [zfs]
    [ 9305.221515]  [<ffffffffc0acf9f6>] ? tsd_hash_search.isra.0+0x46/0xa0 [spl]
    [ 9305.221519]  [<ffffffffc0ad0214>] ? tsd_set+0x2b4/0x500 [spl]
    [ 9305.221520]  [<ffffffffbbe98a82>] ? mutex_lock+0x12/0x30
    [ 9305.221551]  [<ffffffffc0eb9e97>] zil_commit+0x17/0x20 [zfs]
    [ 9305.221582]  [<ffffffffc0eaf2aa>] zfs_fsync+0x7a/0xf0 [zfs]
    [ 9305.221611]  [<ffffffffc0ec5878>] zpl_fsync+0x68/0xa0 [zfs]
    [ 9305.221614]  [<ffffffffbb868b9b>] vfs_fsync_range+0x4b/0xb0
    [ 9305.221616]  [<ffffffffbb868c5d>] do_fsync+0x3d/0x70
    [ 9305.221618]  [<ffffffffbb868ef0>] SyS_fsync+0x10/0x20
    [ 9305.221619]  [<ffffffffbbe9b2b6>] entry_SYSCALL_64_fastpath+0x1e/0xa8
    [ 9305.221621] INFO: task mozStorage #8:5507 blocked for more than 120 seconds.
    [ 9305.221622]       Tainted: P           OE   4.8.0-32-generic #34-Ubuntu
    [ 9305.221623] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
    [ 9305.221624] mozStorage #8   D ffffa23f1573fc18     0  5507  19750 0x00000004
    [ 9305.221625]  ffffa23f1573fc18 00000179907cd000 ffffa240ecf72ac0 ffffa23f944b9c80
    [ 9305.221627]  ffffa240ec957188 ffffa23f15740000 ffffa240ff319200 7fffffffffffffff
    [ 9305.221629]  ffffa23d0ac9a5e0 0000000000000001 ffffa23f1573fc30 ffffffffbbe96b15
    [ 9305.221630] Call Trace:
    [ 9305.221631]  [<ffffffffbbe96b15>] schedule+0x35/0x80
    [ 9305.221633]  [<ffffffffbbe9a2ca>] schedule_timeout+0x22a/0x3f0
    [ 9305.221662]  [<ffffffffc0ebdb50>] ? zio_taskq_member.isra.6+0x90/0x90 [zfs]
    [ 9305.221666]  [<ffffffffc0aca9ae>] ? taskq_dispatch_ent+0x4e/0x120 [spl]
    [ 9305.221668]  [<ffffffffbb6fc511>] ? ktime_get+0x41/0xb0
    [ 9305.221669]  [<ffffffffbbe96304>] io_schedule_timeout+0xa4/0x110
    [ 9305.221673]  [<ffffffffc0acee12>] cv_wait_common+0xb2/0x130 [spl]
    [ 9305.221674]  [<ffffffffbb6c7150>] ? wake_atomic_t_function+0x60/0x60
    [ 9305.221678]  [<ffffffffc0aceee8>] __cv_wait_io+0x18/0x20 [spl]
    [ 9305.221706]  [<ffffffffc0ebfcdd>] zio_wait+0xfd/0x1d0 [zfs]
    [ 9305.221736]  [<ffffffffc0eb9aa6>] zil_commit.part.11+0x3c6/0x7a0 [zfs]
    [ 9305.221740]  [<ffffffffc0acf9f6>] ? tsd_hash_search.isra.0+0x46/0xa0 [spl]
    [ 9305.221768]  [<ffffffffc0eb9e97>] zil_commit+0x17/0x20 [zfs]
    [ 9305.221798]  [<ffffffffc0eaf2aa>] zfs_fsync+0x7a/0xf0 [zfs]
    [ 9305.221829]  [<ffffffffc0ec5878>] zpl_fsync+0x68/0xa0 [zfs]
    [ 9305.221833]  [<ffffffffbb868b9b>] vfs_fsync_range+0x4b/0xb0
    [ 9305.221834]  [<ffffffffbb868c5d>] do_fsync+0x3d/0x70
    [ 9305.221836]  [<ffffffffbb868ef0>] SyS_fsync+0x10/0x20
    [ 9305.221838]  [<ffffffffbbe9b2b6>] entry_SYSCALL_64_fastpath+0x1e/0xa8

ProblemType: Bug
DistroRelease: Ubuntu 16.10
Package: zfs-dkms 0.6.5.8-0ubuntu4.1
ProcVersionSignature: Ubuntu 4.8.0-32.34-generic 4.8.11
Uname: Linux 4.8.0-32-generic x86_64
NonfreeKernelModules: zfs zunicode zcommon znvpair zavl
ApportVersion: 2.20.3-0ubuntu8.2
Architecture: amd64
CurrentDesktop: Unity
Date: Fri Jan  6 10:19:22 2017
InstallationDate: Installed on 2015-12-12 (390 days ago)
InstallationMedia: Ubuntu 15.10 "Wily Werewolf" - Release amd64 (20151021)
SourcePackage: zfs-linux
UpgradeStatus: Upgraded to yakkety on 2016-10-17 (80 days ago)