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This patch adds methods to increase the partition power of an existing object ring without downtime for the users using a 3-step process. Data won't be moved to other nodes; objects using the new increased partition power will be located on the same device and are hardlinked to avoid data movement. 1. A new setting "next_part_power" will be added to the rings, and once the proxy server reloaded the rings it will send this value to the object servers on any write operation. Object servers will now create a hard-link in the new location to the original DiskFile object. Already existing data will be relinked using a new tool in the new locations using hardlinks. 2. The actual partition power itself will be increased. Servers will now use the new partition power to read from and write to. No longer required hard links in the old object location have to be removed now by the relinker tool; the relinker tool reads the next_part_power setting to find object locations that need to be cleaned up. 3. The "next_part_power" flag will be removed. This mostly implements the spec in [1]; however it's not using an "epoch" as described there. The idea of the epoch was to store data using different partition powers in their own namespace to avoid conflicts with auditors and replicators as well as being able to abort such an operation and just remove the new tree. This would require some heavy change of the on-disk data layout, and other object-server implementations would be required to adopt this scheme too. Instead the object-replicator is now aware that there is a partition power increase in progress and will skip replication of data in that storage policy; the relinker tool should be simply run and afterwards the partition power will be increased. This shouldn't take that much time (it's only walking the filesystem and hardlinking); impact should be low therefore. The relinker should be run on all storage nodes at the same time in parallel to decrease the required time (though this is not mandatory). Failures during relinking should not affect cluster operations - relinking can be even aborted manually and restarted later. Auditors are not quarantining objects written to a path with a different partition power and therefore working as before (though they are reading each object twice in the worst case before the no longer needed hard links are removed). Co-Authored-By: Alistair Coles <alistair.coles@hpe.com> Co-Authored-By: Matthew Oliver <matt@oliver.net.au> Co-Authored-By: Tim Burke <tim.burke@gmail.com> [1] https://specs.openstack.org/openstack/swift-specs/specs/in_progress/ increasing_partition_power.html Change-Id: I7d6371a04f5c1c4adbb8733a71f3c177ee5448bb |
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