[PATCH 0/1] Block script performance with shared image files

[Mike Latimer <mlatimer@suse.com>]

As mentioned in a previous thread[1], the hotplug block script suffers
from an exponential performance degredation when attaching shared image
files due to scanning xenstore multiple times.

During the attachment of a loopback mounted image file, the mode of all
curent instances of this device already attached to other domains must be
checked. This requires finding all loopback devices pointing to the inode
of the shared image file, and then comparing the major and minor number of
these devices to the major and minor number of every vbd device found in the
xenstore database. Scanning all of xenstore is performed for every instance
of every loopback device pointing to the same shared image file, and becomes
slower and slower with every additional instance.

Rather than scanning the entire xenstore database for every instance of a
shared loopback device, the attached patch creates a list of the major and
minor numbers from all matching loopback devices. Xenstore is then walked
once, and major and minor numbers from every vbd are checked against the list.

I think this is a reasonable approach to this issue, and the benefits can be
seen with as few as 10 domains sharing the same image file.

Thanks,
Mike

[1]http://lists.xenproject.org/archives/html/xen-devel/2015-09/msg03551.html

Mike Latimer (1):
  tools/hotplug: Scan xenstore once when attaching shared images files

 tools/hotplug/Linux/block | 67 +++++++++++++++++++++++++++++------------------
 1 file changed, 41 insertions(+), 26 deletions(-)

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1.8.4.5