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[Xen-bugs] [Bug 357] New: There is no mechanism to detect block device s

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Subject: [Xen-bugs] [Bug 357] New: There is no mechanism to detect block device setup failure
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Date: Thu, 20 Oct 2005 17:53:25 +0000
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           Summary: There is no mechanism to detect block device setup
           Product: Xen
           Version: unstable
          Platform: All
        OS/Version: Linux
            Status: NEW
          Severity: major
          Priority: P2
         Component: Tools
        AssignedTo: xen-bugs@xxxxxxxxxxxxxxxxxxx
        ReportedBy: aliguori@xxxxxxxxxx
                CC: danms@xxxxxxxxxx

Block devices are setup by scripts in /etc/xen/block-*.  These scripts are
invoked by the hotplug daemon after the block device backend has created the
device.  Xend simply puts some info in the store, and chuggs along.

There are two instances of user-facing failure here.  If a user invokes xm
block-create with a device description that's invalid they won't get any error
message.  Also if there's a failure (such as no remaining loop devices) they
won't get notified of this.

A user will also not realize he has a bad disk configuration on xm create
because it will succeed and the domain will simply time out when trying to
attach the frontend device.

The proper way to handle this is to have the hotplug scripts write a "connected"
node into the backend or an "error" node if there's a failure.  Xend should then
set watch the backend directory for one of these two nodes (timing out after a
certain period of time to detect general failure in hotplug scripts).

This way, we can either alert a user on a failure in the hotplug scripts giving
the script-generated error message, or in the event of a timeout, warn the user
that they're hotplug scripts are not installed right (or not at the required

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