Description of problem:
File /usr/share/ovirt-host-deploy/plugins/ovirt-host-deploy/tune/ovirt-iosched.rules contains a udev rule used to switch the disk scheduler to deadline.

The rule does so using a "RUN+=" command:

ACTION=="add|change", SUBSYSTEM=="block", ENV{DEVTYPE}=="disk", RUN+="/bin/sh -c 'echo deadline > /sys/${DEVPATH}/queue/scheduler'"

We are requesting the rule to do so using the sysfs attribute instead:

ACTION=="add|change", SUBSYSTEM=="block", ENV{DEVTYPE}=="disk", ATTR{queue/scheduler}="noop"

The reason for this request is twofold:
- it's probably cheaper to request udev to set the sysfs attribute directly than it is to run a shell command.
- rules supplied by hardware vendors already use sysfs attributes, which introduces the problem described below.


Some vendors recommend different settings, for example:

  https://support.purestorage.com/Solutions/Operating_Systems/Linux/Linux_Recommended_Settings

  ACTION=="add|change", KERNEL=="sd*[!0-9]", SUBSYSTEM=="block", ENV{ID_VENDOR}=="PURE", ATTR{queue/scheduler}="noop"

Taking the above example, creating a "99-*.rules" file with the rule above should override the rule included in file "ovirt-iosched.rules" which ends up deployed in hosts as "12-ovirt-iosched.rules".

BUT while udev honours the numeric order determined by the filenames, udev seems to apply sysfs attributes before running commands.
As a result, the command RUN+="/bin/sh -c 'echo deadline > /sys/${DEVPATH}/queue/scheduler'" in file "12-ovirt-iosched.rules" is executed AFTER the sysfs ATTR{queue/scheduler}="noop" in file "99-*.rules":

# udevadm --debug test /block/sdi 2>&1 | grep scheduler
udev_rules_apply_to_event: RUN '/bin/sh -c 'echo deadline > /sys/${DEVPATH}/queue/scheduler'' /etc/udev/rules.d/12-ovirt-iosched.rules:5
udev_rules_apply_to_event: ATTR '/sys/devices/platform/host6/session1/target6:0:0/6:0:0:7/block/sdi/queue/scheduler' writing 'noop' /etc/udev/rules.d/99-pure-storage.rules:1
udevadm_test: run: '/bin/sh -c 'echo deadline > /sys/${DEVPATH}/queue/scheduler''


Version-Release number of selected component (if applicable):
ovirt-host-deploy-1.4.1-1.el6ev

How reproducible:
Always

Steps to Reproduce:
1. Create a udev rule file which sets the disk elevator to a value other than 'deadline'
2. Ensure that the rule in the new file uses ATTR{queue/scheduler}= to set the sysfs attribute.
3. Ensure that the number in the new rule file is higher than 12 to allow it to override "12-ovirt-iosched.rules".

Actual results:
'deadline' disk scheduler is applied to all devices, including those matched by the new custom rule file.
udev runs commands after sysfs attribute changes.

Expected results:
custom scheduler is applied to devices matched by the new rule file.
"12-ovirt-iosched.rules" uses sysfs attribute to set all devices to 'deadline' disk scheduler, thus allowing other rules that use sysfs attribute to override the disk scheduler as per the comments in the file:

"# Settings may be overridden by applying subsequent rules."