Hello Colin,
  Thank you for responding. To Answer your questions:
  1. I have two external drives, sdb and sdc, in USB 3  Orico enclosure. The 
drives are mirrored.
  2. Atom D525 with 4GiB RAM. The attached screenshot shows "vmstat 5 5" output 
while VLC is playing a movie. It shows 1.5GiB of free memory, no swap used.
  3. Screenshot also shows output from "iostat -mx", and that I manually set 
the spl_kmem_cache_slab_limit to zero as suggested.

  I played a movie which paused repeatedly for 30 seconds and one time
for several minutes. When the pause occurs, VLC is completely hung until
the requested I/O completes. The end of pause is heralded by the I/O
activity LED on the drive enclosure.

  I want to emphasize/re-iterate a couple of points from my original post:
  1. This is not a VLC problem. I can recreate the problem with any app (cp, 
rsyn, etc) that reads sequentially from the ZFS file system.
  2. I do not believe this is a drive/latency problem: I can dd the content of 
sdb and sdc (to /dev/null) without any hang.

  Regards, Colin.

** Attachment added: "spl_kmem_cache_slab_limit, vmstat, and iostat -mx"

You received this bug notification because you are a member of Kernel
Packages, which is subscribed to zfs-linux in Ubuntu.

  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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