[Pkg-ganeti-devel] [ganeti] 08/165: Add design document on allocation changes for dedicated

[Apollon Oikonomopoulos]

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commit fb6d0a7d86429494f43fab69d1157e2ee0d757ce
Author: Klaus Aehlig <aehlig at google.com>
Date:   Wed Jan 21 10:42:54 2015 +0100

    Add design document on allocation changes for dedicated
    
    Add a design document detailing how allocation is changed
    on dedicated clusters in order to avoid not being able
    to put a big instance on the cluster as all nodes are taken
    up by small instances.
    
    Signed-off-by: Klaus Aehlig <aehlig at google.com>
    Reviewed-by: Petr Pudlak <pudlak at google.com>
---
 Makefile.am                         |  1 +
 doc/design-dedicated-allocation.rst | 87 +++++++++++++++++++++++++++++++++++++
 doc/design-draft.rst                |  1 +
 3 files changed, 89 insertions(+)

diff --git a/Makefile.am b/Makefile.am
index 736e0d2..4744e15 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -651,6 +651,7 @@ docinput = \
 	doc/design-cpu-pinning.rst \
 	doc/design-cpu-speed.rst \
 	doc/design-daemons.rst \
+        doc/design-dedicated-allocation.rst \
 	doc/design-device-uuid-name.rst \
 	doc/design-disk-conversion.rst \
 	doc/design-disks.rst \
diff --git a/doc/design-dedicated-allocation.rst b/doc/design-dedicated-allocation.rst
new file mode 100644
index 0000000..b0a81fc
--- /dev/null
+++ b/doc/design-dedicated-allocation.rst
@@ -0,0 +1,87 @@
+=================================
+Allocation for Partitioned Ganeti
+=================================
+
+.. contents:: :depth: 4
+
+
+Current state and shortcomings
+==============================
+
+The introduction of :doc:`design-partitioned` allowed to
+dedicate resources, in particular storage, exclusively to
+an instance. The advantage is that such instances have
+guaranteed latency that is not affected by other
+instances. Typically, those instances are created once
+and never moved. Also, typically large chunks (full, half,
+or quarter) of a node are handed out to individual
+partitioned instances.
+
+Ganeti's allocation strategy is to keep the cluster as
+balanced as possible. In particular, as long as empty nodes
+are available, new instances, regardless of their size,
+will be placed there. Therefore, if a couple of small
+instances are placed on the cluster first, it will no longer
+be possible to place a big instance on the cluster despite
+the total usage of the cluster being low.
+
+
+Proposed changes
+================
+
+We propose to change the allocation strategy of hail for
+node groups that have the ``exclusive_storage`` flag set,
+as detailed below; nothing will be changed for non-exclusive
+node groups. The new strategy will try to keep the cluster
+as available for new instances as possible.
+
+Dedicated Allocation Metric
+---------------------------
+
+The instance policy is a set of intervals in which the resources
+of the instance have to be. Typical choices for dedicated clusters
+have disjoint intervals with the same monotonicity in every dimension.
+In this case, the order is obvious. In order to make it well-defined
+in every case, we specify that we sort the intervals by the lower
+bound of the disk size. This is motivated by the fact that disk is
+the most critical aspect of partitioned Ganeti.
+
+For a node the *allocation vector* is the vector of, for each
+instance policy interval in decreasing order, the number of
+instances minimally compliant with that interval that still
+can be placed on that node. For the drbd template, it is assumed
+that all newly placed instances have new secondaries.
+
+The *lost-allocations vector* for an instance on a node is the
+difference of the allocation vectors for that node before and
+after placing that instance on that node. Lost-allocation vectors
+are ordered lexicographically, i.e., a loss of an allocation
+larger instance size dominates loss of allocations of smaller
+instance sizes.
+
+If allocating in a node group with ``exclusive_storage`` set
+to true, hail will try to minimise the pair of the lost-allocations
+vector and the remaining disk space on the node afer, ordered
+lexicographically.
+
+Example
+-------
+
+Consider the already mentioned scenario were only full, half, and quarter
+nodes are given to instances. Here, for the placement of a
+quarter-node--sized instance we would prefer a three-quarter-filled node (lost
+allocations: 0, 0, 1 and no left overs) over a quarter-filled node (lost
+allocations: 0, 0, 1 and half a node left over)
+over a half-filled node (lost allocations: 0, 1, 1) over an empty
+node (lost allocations: 1, 1, 1). A half-node sized instance, however,
+would prefer a half-filled node (lost allocations: 0, 1, 2 and no left-overs)
+over a quarter-filled node (lost allocations: 0, 1, 2 and a quarter node left
+over) over an empty node (lost allocations: 1, 1, 2).
+
+Note that the presence of additional policy intervals affects the preferences
+of instances of other sizes as well. This is by design, as additional available
+instance sizes make additional remaining node sizes attractive. If, in the
+given example, we would also allow three-quarter-node--sized instances, for
+a quarter-node--sized instance it would now be better to be placed on a
+half-full node (lost allocations: 0, 0, 1, 1) than on a quarter-filled
+node (lost allocations: 0, 1, 0, 1).
diff --git a/doc/design-draft.rst b/doc/design-draft.rst
index 01f63ce..2b24ce4 100644
--- a/doc/design-draft.rst
+++ b/doc/design-draft.rst
@@ -26,6 +26,7 @@ Design document drafts
    design-network2.rst
    design-configlock.rst
    design-multi-storage-htools.rst
+   design-dedicated-allocation.rst
 
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