File xendomains-wait-disks.README.md of Package xen

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File xendomains-wait-disks.README.md of Package xen (Revision 8ae33324fb43475f6644babb6702e020)

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# xen-tools-xendomains-wait-disk

[xendomains.service](https://github.com/xen-project/xen/blob/RELEASE-4.13.0/tools/hotplug/Linux/systemd/xendomains.service.in) has problems
with disks that appear only later in boot process (or even after booting is complete). This project creates a service that
loops over all disks that domU will use and wait for them to appear.

xendomains-wait-disk.service launches a script that reads both /etc/xen/auto/ configurations and /var/lib/xen/save/ dumps.
From those files, it extracts which disks are needed for all domU that will be started (respecting /etc/sysconfig/xendomains
settings). After that, it simply loops waiting for those disks to appear. There is a timeout (5 min) configured in
xendomains-wait-disk.service that prevents it to block booting process forever.

There are two known cases where this project is useful:

## degraded mdadm RAID

mdadm RAID are assembled by [udev rules](https://github.com/neilbrown/mdadm/blob/master/udev-md-raid-assembly.rules). 
However, it is only assembled when it is healthy. When a member is still missing, it starts a [timer](https://github.com/neilbrown/mdadm/blob/master/systemd/mdadm-last-resort%40.timer) that will try to assemble the RAID anyway after 30s, even if degraded. This timer does not block xendomains to be started. So, if a domU is depending on a MD RAID that is degraded (i.e. RAID 1 missing one disk), xendomains.service will be started before those 30s passed and that domU will fail.

An alternative solution would be to add extra hard dependencies to xendomains.service for each required disk (Require=xxx.device). However, this solution introduces another bigger problem. Before, if a single RAID is degraded, only the domU that depends on it will fail. With Require=xxx.device, xendomains will never start if
a RAID could not be assembled even after 30s (i.e. RAID5 with two missing disks).

With xendomains-wait-disk.service, xendomains.service will be blocked up to 5 min waiting for those MD RAID used by domUs. If it fails, xendomains.service
continues anyway.

## iSCSI disks

domU that uses iSCSI disk (mapped by host OS) also fails to start during boot. open-iscsi.service returns before it connect to the remote target and rescan
iscsi disks. As in mdadm RAID case, xendomains.service is started and domU that depends on iSCSI disks will fail.

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