fstrim slows down mixed ssd/hdd considerable

[Michael Vogt]

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.

[Martin Pitt (pitti)]

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.

[Launchpad Janitor (janitor)]

Status changed to 'Confirmed' because the bug affects multiple users.