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[Martin Pitt <martin.pitt@xxxxxxxxxx>]

It's not really that surprising -- it needs to iterate over the mounts,
find free blocks, and tell the disk controller to trim them; on a
developer machine where there is usually a lot of data turnaround over a
week, this usually has to process dozens of GB.

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https://bugs.launchpad.net/bugs/1393335

Title:
  fstrim slows down mixed ssd/hdd considerable

Status in “util-linux” package in Ubuntu:
  New

Bug description:
  I have a ssd/hdd system where the ssd is used for the system and most
  of home is on a large hdd.

  I noticed that the weekly fstrim cron job slows the system down considerable, including e.g. firefox and
  the desktop. iotop reports:
  """
  otal DISK READ :       0.00 B/s | Total DISK WRITE :       0.00 B/s
  Actual DISK READ:       0.00 B/s | Actual DISK WRITE:       7.71 K/s
   TID  PRIO  USER     DISK READ  DISK WRITE  SWAPIN     IO>    COMMAND
  24504 be/4 root        0.00 B/s    0.00 B/s  0.00 % 99.99 % fstrim --all
  """

  So hardly any real io activity but still massive slowdown with
  3.16.0-24-generic.

  E.g. a dpkg operation while fstrim is running:
  """
  # time dpkg -i 2vcard_0.5-3_all.deb
  real	0m8.221s
  user	0m0.895s
  sys	0m0.671s

  # time dpkg -i 2vcard_0.5-3_all.deb
  real	0m8.215s
  user	0m0.895s
  sys	0m0.705s

  # killall fstrim
  # time dpkg -i 2vcard_0.5-3_all.deb
  real	0m2.023s
  user	0m0.992s
  sys	0m0.703s

  # time dpkg -i 2vcard_0.5-3_all.deb
  real	0m1.709s
  user	0m0.958s
  sys	0m0.738s
  """

  This may of course be simply a kernel problem but I do get similar
  result for both 3.16 and 3.13.

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