Category Archives: ceph

An algorithm to fix uneven CRUSH distributions in Ceph

The current CRUSH implementation in Ceph does not always provide an even distribution. The most common cause of unevenness is when only a few thousands PGs, or less, are mapped. This is not enough samples and the variations can be … Continue reading

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Ceph space lost due to overweight CRUSH items

When a CRUSH bucket contains five Ceph OSDs with the following weights: weight osd.0 5 osd.1 1 osd.2 1 osd.3 1 osd.4 1 20% of the space in osd.0 will never be used by a pool with two replicas. The … Continue reading

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Ceph full ratio and uneven CRUSH distributions

A common CRUSH rule in Ceph is step chooseleaf firstn 0 type host meaning Placement Groups (PGs) will place replicas on different hosts so the cluster can sustain the failure of any host without losing data. The missing replicas are … Continue reading

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Improving PGs distribution with CRUSH weight sets

In a Ceph cluster with a single pool of 1024 Placement Groups (PGs), the PG distribution among devices will not be as expected. (see Predicting Ceph PG placement for details about this uneven distribution). In the following, the difference between … Continue reading

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Faster Ceph CRUSH computation with smaller buckets

The CRUSH function maps Ceph placement groups (PGs) and objects to OSDs. It is used extensively in Ceph clients and daemons as well as in the Linux kernel modules and its CPU cost should be reduced to the minimum. It … Continue reading

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Predicting Ceph PG placement

When creating a new Ceph pool, deciding for the number of PG requires some thinking to ensure there are a few hundred PGs per OSD. The distribution can be verified with crush analyze as follows: $ crush analyze –rule data … Continue reading

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How many objects will move when changing a crushmap ?

After a crushmap is changed (e.g. addition/removal of devices, modification of weights or tunables), objects may move from one device to another. The crush compare command can be used to show what would happen for a given rule and replication … Continue reading

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Predicting which Ceph OSD will fill up first

When a device is added to Ceph, it is assigned a weight that reflects its capacity. For instance if osd.1 is a 1TB disk, its weight will be 1.0 and if osd.2 is a 4TB disk, its weight will be … Continue reading

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Testing Ceph with ARMv8 OpenStack instances

The Ceph integration tests can be run on ARMv8 (aka arm64 or aarch64) OpenStack instances on CloudLab or Runabove. When logged in CloudLab an OpenStack cluster suitable for teuthology must be created. To start an experiment click Change Profile to … Continue reading

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teuthology forensics with git, shell and paddles

When a teuthology integration test for Ceph fails, the results are analyzed to find the source of the problem. For instance the upgrade suite: pool_create failed with error -4 EINTR issue was reported early October 2015, with multiple integration job … Continue reading

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