1880351f1a
Imagine a 3-zone ring, and consider a partition in that ring with replicas placed as follows: * replica 0 is on device A (zone 2) * replica 1 is on device B (zone 1) * replica 2 is on device C (zone 2) Further, imagine that there are zero parts_wanted in all of zone 3; that is, zone 3 is completely full. However, zones 1 and 2 each have at least one parts_wanted on at least one device. When the ring builder goes to gather replicas to move, it gathers replica 0 because there are three zones available, but the replicas are only in two of them. Then, it places replica 0 in zone 1 or 2 somewhere because those are the only zones with parts_wanted. Notice that this does *not* do anything to spread the partition out better. Then, on the next rebalance, replica 0 gets picked up and moved (again) but doesn't improve its placement (again). If your builder has min_part_hours > 0 (and it should), then replicas 1 and 2 cannot move at all. A coworker observed the bug because a customer had such a partition, and its replica 2 was on a zero-weight device. He thought it odd that a zero-weight device should still have one partition on it despite the ring having been rebalanced dozens of times. Even if you don't have zero-weight devices, having a bunch of partitions trade places on each rebalance isn't particularly good. Note that this only happens with an unbalanceable ring; if the ring *can* balance, the gathered partitions will swap places, but they will get spread across more zones, so they won't get gathered up again on the next rebalance. Change-Id: I8f44f032caac25c44778a497dedf23f5cb61b6bb Closes-Bug: 1400083
Swift
A distributed object storage system designed to scale from a single machine to thousands of servers. Swift is optimized for multi-tenancy and high concurrency. Swift is ideal for backups, web and mobile content, and any other unstructured data that can grow without bound.
Swift provides a simple, REST-based API fully documented at http://docs.openstack.org/.
Swift was originally developed as the basis for Rackspace's Cloud Files and was open-sourced in 2010 as part of the OpenStack project. It has since grown to include contributions from many companies and has spawned a thriving ecosystem of 3rd party tools. Swift's contributors are listed in the AUTHORS file.
Docs
To build documentation install sphinx (pip install sphinx), run
python setup.py build_sphinx, and then browse to /doc/build/html/index.html.
These docs are auto-generated after every commit and available online at
http://docs.openstack.org/developer/swift/.
For Developers
The best place to get started is the "SAIO - Swift All In One". This document will walk you through setting up a development cluster of Swift in a VM. The SAIO environment is ideal for running small-scale tests against swift and trying out new features and bug fixes.
You can run unit tests with .unittests and functional tests with
.functests.
If you would like to start contributing, check out these notes to help you get started.
Code Organization
- bin/: Executable scripts that are the processes run by the deployer
- doc/: Documentation
- etc/: Sample config files
- swift/: Core code
- account/: account server
- common/: code shared by different modules
- middleware/: "standard", officially-supported middleware
- ring/: code implementing Swift's ring
- container/: container server
- obj/: object server
- proxy/: proxy server
- test/: Unit and functional tests
Data Flow
Swift is a WSGI application and uses eventlet's WSGI server. After the
processes are running, the entry point for new requests is the Application
class in swift/proxy/server.py. From there, a controller is chosen, and the
request is processed. The proxy may choose to forward the request to a back-
end server. For example, the entry point for requests to the object server is
the ObjectController class in swift/obj/server.py.
For Deployers
Deployer docs are also available at http://docs.openstack.org/developer/swift/. A good starting point is at http://docs.openstack.org/developer/swift/deployment_guide.html
You can run functional tests against a swift cluster with .functests. These
functional tests require /etc/swift/test.conf to run. A sample config file
can be found in this source tree in test/sample.conf.
For Client Apps
For client applications, official Python language bindings are provided at http://github.com/openstack/python-swiftclient.
Complete API documentation at http://docs.openstack.org/api/openstack-object-storage/1.0/content/
For more information come hang out in #openstack-swift on freenode.
Thanks,
The Swift Development Team