openstack/openstack-manuals
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Adding notion of overload to swift-ring-builder

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Adding overload concept information to ring builder

Change-Id: Id801f587c02da2b8f083677ad96f61826d89579d
Closes-bug: #1409127
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asettle
2015-09-08 14:28:54 +10:00
parent 7af308a814
commit de760a6c88
1 changed files with 38 additions and 0 deletions
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38
doc/admin-guide-cloud/source/objectstorage_ringbuilder.rst
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@@ -51,6 +51,44 @@ a partition will have more than one replica on a device.
``array('H')`` is used for memory conservation as there may be millions ``array('H')`` is used for memory conservation as there may be millions
of partitions. of partitions.
Overload
~~~~~~~~
The ring builder tries to keep replicas as far apart as possible while
still respecting device weights. When it can not do both, the overload
factor determines what happens. Each device takes an extra
fraction of its desired partitions to allow for replica dispersion;
after that extra fraction is exhausted, replicas are placed closer
together than optimal.
The overload factor lets the operator trade off replica
dispersion (durability) against data dispersion (uniform disk usage).
The default overload factor is 0, so device weights are strictly
followed.
With an overload factor of 0.1, each device accepts 10% more
partitions than it otherwise would, but only if it needs to maintain
partition dispersion.
For example, consider a 3-node cluster of machines with equal-size disks;
node A has 12 disks, node B has 12 disks, and node C has
11 disks. The ring has an overload factor of 0.1 (10%).
Without the overload, some partitions would end up with replicas only
on nodes A and B. However, with the overload, every device can accept
up to 10% more partitions for the sake of dispersion. The
missing disk in C means there is one disk's worth of partitions
to spread across the remaining 11 disks, which gives each
disk in C an extra 9.09% load. Since this is less than the 10%
overload, there is one replica of each partition on each node.
However, this does mean that the disks in node C have more data
than the disks in nodes A and B. If 80% full is the warning
threshold for the cluster, node C's disks reach 80% full while A
and B's disks are only 72.7% full.
Replica counts Replica counts
~~~~~~~~~~~~~~ ~~~~~~~~~~~~~~
To support the gradual change in replica counts, a ring can have a real To support the gradual change in replica counts, a ring can have a real