The iostat -mx shows that the /dev/sdb /dev/sdc devices have a very high
read I/O latency of ~80-110 milliseconds which may be the underlying

I suggest seeing how long it takes to read data from these raw devices
to sanity check their maximum speed.  One can do this by doing:

echo 3 | sudo tee /proc/sys/vm/drop_caches
dd if=/dev/sdb of=/dev/null bs=512 count=1048576

..and wait for 512MB of data to be read and see what the read speed is
of the raw device.

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  zfs frequently hangs (up to 30 secs) during sequential read

Status in zfs-linux package in Ubuntu:

Bug description:
  Updated to artful (17.10) yesterday. Trying to read (play video) from 
mirrored ZFS disks from an external USB 3 enclosure. Zpool is defined as:
  root@hetty:/home/crlb# zpool status
    pool: storage
   state: ONLINE
    scan: resilvered 20K in 0h0m with 0 errors on Fri Oct 20 18:38:49 2017

        NAME        STATE     READ WRITE CKSUM
        storage     ONLINE       0     0     0
          mirror-0  ONLINE       0     0     0
            sdb     ONLINE       0     0     0
            sdc     ONLINE       0     0     0

  errors: No known data errors

  Found that I could re-create the problem with:

   rsync -av --progress <zfs_file> <internal_ext4_path>

  Also found that:

    dd if=/dev/sdX of=/dev/null status=progress bs=1024 count=10000000

  Where "X" is either "b" or "c" does not hang.


  root@hetty:/home/crlb# apt list --installed | grep -i zfs
  libzfs2linux/artful,now amd64 [installed,automatic]
  zfs-zed/artful,now amd64 [installed,automatic]
  zfsutils-linux/artful,now amd64 [installed]

  Help please.

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