1. what is your ZFS configuration (number of drives?)
2. size of your system (total memory, free memory).
3. Do your drive(s) suffer from large I/O latencies?
   - what is the output from iostat -mx

You may possibly get some benefit from setting spl_kmem_cache_slab_limit
to zero as a module parameter for the spl ZoL kernel module. This avoids
SLUB slab merging entirely and maybe helpful from anecdotal evidence
I've seen.

** Changed in: zfs-linux (Ubuntu)
       Status: New => Triaged

** Changed in: zfs-linux (Ubuntu)
   Importance: Undecided => Medium

** Changed in: zfs-linux (Ubuntu)
     Assignee: (unassigned) => Colin Ian King (colin-king)

** Changed in: zfs-linux (Ubuntu)
       Status: Triaged => Incomplete

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https://bugs.launchpad.net/bugs/1725859

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

Status in zfs-linux package in Ubuntu:
  Incomplete

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
  config:

        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
  root@hetty:/home/crlb#

  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.

  Installed:

  root@hetty:/home/crlb# apt list --installed | grep -i zfs
  libzfs2linux/artful,now 0.6.5.11-1ubuntu3 amd64 [installed,automatic]
  zfs-zed/artful,now 0.6.5.11-1ubuntu3 amd64 [installed,automatic]
  zfsutils-linux/artful,now 0.6.5.11-1ubuntu3 amd64 [installed]
  root@hetty:/home/crlb#

  Help please.

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