mirror of https://github.com/ceph/ceph
179 lines
6.5 KiB
ReStructuredText
179 lines
6.5 KiB
ReStructuredText
===========================
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Rados Gateway Data Layout
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===========================
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Although the source code is the ultimate guide, this document helps
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new developers to get up to speed with the implementation details.
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Introduction
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------------
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Swift offers something called a container, that we use interchangeably with
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the term bucket. One may say that RGW's buckets implement Swift containers.
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This document does not consider how RGW operates on these structures,
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e.g. the use of encode() and decode() methods for serialization and so on.
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Conceptual View
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---------------
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Although RADOS only knows about pools and objects with their xattrs and
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omap[1], conceptually RGW organizes its data into three different kinds:
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metadata, bucket index, and data.
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* Metadata
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We have 3 'sections' of metadata: 'user', 'bucket', and 'bucket.instance'.
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You can use the following commands to introspect metadata entries:
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$ radosgw-admin metadata list
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$ radosgw-admin metadata list bucket
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$ radosgw-admin metadata list bucket.instance
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$ radosgw-admin metadata list user
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$ radosgw-admin metadata get bucket:<bucket>
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$ radosgw-admin metadata get bucket.instance:<bucket>:<bucket_id>
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$ radosgw-admin metadata get user:<user> # or set
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user: Holds user information
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bucket: Holds a mapping between bucket name and bucket instance id
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bucket.instance: Holds bucket instance information[2]
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Every metadata entry is kept on a single rados object.
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See below for implementation defails.
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Note that the metadata is not indexed. When listing a metadata section we do a
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rados pgls operation on the containing pool.
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* Bucket Index
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It's a different kind of metadata, and kept separately. The bucket index holds
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a key-value map in rados objects. By default it is a single rados object per
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bucket, but it is possible since Hammer to shard that map over multiple rados
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objects. The map itself is kept in omap, associated with each rados object.
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The key of each omap is the name of the objects, and the value holds some basic
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metadata of that object -- metadata that shows up when listing the bucket.
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Also, each omap holds a header, and we keep some bucket accounting metadata
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in that header (number of objects, total size, etc.).
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Note that we also hold other information in the bucket index, and it's kept in
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other key namespaces. We can hold the bucket index log there, and for versioned
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objects there is more information that we keep on other keys.
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* Data
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Objects data is kept in one or more rados objects for each rgw object.
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Object Lookup Path
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------------------
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When accessing objects, ReST APIs come to RGW with three parameters:
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account information (access key in S3 or account name in Swift),
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bucket or container name, and object name (or key). At present, RGW only
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uses account information to find out the user ID and for access control.
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Only the bucket name and object key are used to address the object in a pool.
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The user ID in RGW is a string, typically the actual user name from the user
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credentials and not a hashed or mapped identifier.
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When accessing a user's data, the user record is loaded from an object
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"<user_id>" in pool ".users.uid".
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Bucket names are represented directly in the pool ".rgw". Bucket record is
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loaded in order to obtain so-called marker, which serves as a bucket ID.
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The object is located in pool ".rgw.buckets". Object name is "<marker>_<key>",
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for example "default.7593.4_image.png", where the marker is "default.7593.4"
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and the key is "image.png". Since these concatenated names are not parsed,
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only passed down to RADOS, the choice of the separator is not important and
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causes no ambiguity. For the same reason, slashes are permitted in object
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names (keys).
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It is also possible to create multiple data pools and make it so that
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different users buckets will be created in different rados pools by default,
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thus providing the necessary scaling. The layout and naming of these pools
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is controlled by a 'policy' setting.[3]
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An RGW object may consist of several RADOS objects, the first of which
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is the head that contains the metadata, such as manifest, ACLs, content type,
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ETag, and user-defined metadata. The metadata is stored in xattrs.
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The head may also contain up to 512 kilobytes of object data, for efficiency
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and atomicity. The manifest describes how each object is laid out in RADOS
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objects.
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Bucket and Object Listing
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-------------------------
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Buckets that belong to a given user are listed in an omap of an object named
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"<user_id>.buckets" (for example, "foo.buckets") in pool ".users.uid".
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These objects are accessed when listing buckets, when updating bucket
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contents, and updating and retrieving bucket statistics (e.g. for quota).
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See the user-visible, encoded class 'cls_user_bucket_entry' and its
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nested class 'cls_user_bucket' for the values of these omap entires.
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These listings are kept consistent with buckets in pool ".rgw".
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Objects that belong to a given bucket are listed in a bucket index,
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as discussed in sub-section 'Bucket Index' above. The default naming
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for index objects is ".dir.<marker>" in pool ".rgw.buckets.index".
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Footnotes
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---------
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[1] Omap is a key-value store, associated with an object, in a way similar
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to how Extended Attributes associate with a POSIX file. An object's omap
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is not physically located in the object's storage, but its precise
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implementation is invisible and immaterial to RADOS Gateway.
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In Hammer, one LevelDB is used to store omap in each OSD.
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[2] Before the Dumpling release, the 'bucket.instance' metadata did not
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exist and the 'bucket' metadata contained its information. It is possible
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to encounter such buckets in old installations.
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[3] In Infernalis, a pending commit exists that removes the need of prefixing
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all the rgw system pools with a period, and also renames all of these pools.
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See Github pull request #4944 "rgw noperiod".
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Appendix: Compendum
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-------------------
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Known pools:
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.rgw.root
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Unspecified region, zone, and global information records, one per object.
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.rgw.control
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notify.<N>
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.rgw
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<bucket>
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.bucket.meta.<bucket>:<marker>
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.rgw.gc
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gc.<N>
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.users.uid
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Contains _both_ per-user information (RGWUserInfo) in "<user>" objects
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and per-user bucket lists in omaps of "<user>.buckets" objects.
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.users.email
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Unimportant
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.users
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47UA98JSTJZ9YAN3OS3O
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It's unclear why user ID is not used to name objects in this pool.
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.users.swift
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test:tester
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.rgw.buckets.index
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Objects are named ".dir.<marker>", each contains a bucket index.
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.rgw.buckets
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default.7593.4__shadow_.488urDFerTYXavx4yAd-Op8mxehnvTI_1
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<marker>_<key>
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An example of a marker would be "default.16004.1" or "default.7593.4".
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