** Description changed:
[Impact]
A user reports that cachefilesd will spin at 100% of a cpu when started
on a filesystem where the free space is less than the bcull threshold
and culling the cache is insufficient to free up space.
Investigation shows that this is because cachefilesd detects that
culling is required, tries to cull, and does not realise that culling
cannot free up enough space, so just keeps retrying.
[Test Case]
- I created a VM with a spare disk, mounted it to /raid (or whatever).
- I changed the /etc/default/cachefilesd to start at boot, filled the /raid
filesystem to over the bcull threshold, and started cachefilesd. When running
top, the cachefilesd process is at the top using approx 100% of 1 CPU. It
appears to be trying to free up space in the cache (that is not even used)
because the filesystem is over the threshold for culling.
+ Create a trusty or xenial VM, and install cachefilesd. Using either a real
disk or loopback image, create a ext4 filesystem, and edit fstab to mount it at
/var/cache/fscache, e.g.:
+ $ sudo dd if=/dev/zero of=/cache.img bs=1024m count=1024
+ $ sudo losetup -f /cache.img
+ $ sudo losetup -a
+ $ sudo mkfs.ext4 /dev/loop0 (note, adjust loop0 if needed)
+
+ edit fstab e.g.:
+ $ grep fscache /etc/fstab
+ /cache.img /var/cache/fscache ext4 defaults,loop,user_xattr 0 0
+
+ It's important to include the 'user_xattr' option as cachefilesd
+ requires that.
+
+ stop the cachefilesd service and move the fscache contents:
+ $ sudo service cachefilesd stop
+ $ cd /var/cache
+ $ sudo mkdir fscache2
+ $ sudo mv -vf fscache/* fscache2/
+ $ sudo mount fscache
+ $ sudo mv -vf fscache2/* fscache/
+ $ sudo rmdir fscache2
+
+ create a file to fill up the fscache space, e.g.:
+ $ sudo dd if=/dev/zero of=/var/cache/fscache/largefile.txt bs=1024k count=850
+ $ df /var/cache/fscache
+ Filesystem 1K-blocks Used Available Use% Mounted on
+ /dev/loop0 999320 922896 7612 100% /var/cache/fscache
+
+ edit /etc/default/cachefilesd to uncomment 'RUN=yes', e.g.:
+ $ grep RUN /etc/default/cachefilesd
+ RUN=yes
+
+ reboot, or just restart cachefilesd service
+ $ sudo service cachefilesd start
+
+ check top
+ $ top
+
+ cachefilesd should be spinning, using 100% (or as much as it can) cpu
+ time.
[Regression Potential]
The patch makes changes to how cachefilesd detects if it should sleep
or cull, so regressions would be in the area of cachefilesd spinning
instead of sleeping (which is what it does now) or sleeping instead
of culling.
However the patch is small and easily understood and backports with
minimal effort.
[Other Info]
This is fixed upstream in 0.10.6:
* Wed Feb 3 2016 David Howells <dhowe...@redhat.com> 0.10.6-1
...
- Suspend culling when cache space is short and cache objects are pinned.
The particular patch is ce353f5b6b5b ("cachefilesd can spin when disk
space is short.")
Since bionic has version 0.10.10-0.1, this fix is needed only for xenial
and trusty.
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https://bugs.launchpad.net/bugs/1810372
Title:
Infinite busy-loop trying to cull when cache space is short
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