On 13/09/2017 06:08, Luiz Angelo Daros de Luca wrote:
Hello,
I'm facing a problem with Xen machines that depends on MD devices.
As I'm new to systemd world, I might not be seeing the clean solution.
That's why I'm asking for an advice.
MD devices are automatically detect by udev. If device state is safe,
/dev/md/myraid is started. However, if the state is not safe (like a
raid with a missing disk), udev starts mdadm-last-resort@.timer that
tries to start the device anyway after 30s. As mdadm-last-resort
conflicts with the device presence, it will not run if the required
disk appears before 30s. Even with mdadm-last-resort running, MD
device might still be usable, although degraded.
Xen VM are started by xendomains.service, that simply calls a shell
script like in SysV times. It start a bunch of VM in sequence.
xendomains.service has only generic dependencies that let it run at a
very early stage.
I would cut that script in multiple services... but you mention doing
that below, so I'm just saying I think it's the right way to go :)
Now the problem: If for any reason, a MD device takes some seconds to
appear (or even 30s as the last resort), xendomains will fail to start
any machine that depends on that MD device.
I'm extending manually xendomains.service to depend on a series of MD
devices, that fixed the start order problem. However, I created new
problems. First I had to frequently regenerate those "Requires" as
machines are frequently started/migrated between hosts (I also
consider using systemd generators). But worse, whenever a single MD
device permanently fails, xendomains is never started and all those VM
that does not use the failed MD never start.
I though that maybe I could use instances (xendomains@vm1.service) to
launch VM individually, each of them depending on those devices it
uses. However, these instances would have to be dynamically generated
based on its configuration (systemd generators from
/etc/xen/vm/xxx.cfg?), either on boot, shutdown or simply
periodically. The stop procedure will still be the same, calling the
SysV script as systemd will not know about VM (re)started after boot.
you could create a template (xendomains@.service) and use drop-ins to do
per-instance overrides
(/etc/systemd/system/xendomains@vm1.service.d/append.conf) that would
allow you to individualize each domain while keeping the common parts
I would find a way to prevent a domain with no corresponding to start
(maybe by not defining a mandatory key in the template) to make sure no
rogue domains are created...
I even though about simply create an alternative xendomains.service
that does not depend on any MD device and launch it using a systemd
timer if the xendomains that depends on MD devices isn't started after
30s. It looks ugly but it might work.
Regards,
--
Luiz Angelo Daros de Luca
luizl...@gmail.com <mailto:luizl...@gmail.com>
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