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diff --git a/doc/radosgw/layout.rst b/doc/radosgw/layout.rst new file mode 100644 index 00000000..04a525b6 --- /dev/null +++ b/doc/radosgw/layout.rst @@ -0,0 +1,208 @@ +=========================== + Rados Gateway Data Layout +=========================== + +Although the source code is the ultimate guide, this document helps +new developers to get up to speed with the implementation details. + +Introduction +------------ + +Swift offers something called a container, that we use interchangeably with +the term bucket. One may say that RGW's buckets implement Swift containers. + +This document does not consider how RGW operates on these structures, +e.g. the use of encode() and decode() methods for serialization and so on. + +Conceptual View +--------------- + +Although RADOS only knows about pools and objects with their xattrs and +omap[1], conceptually RGW organizes its data into three different kinds: +metadata, bucket index, and data. + +Metadata +^^^^^^^^ + +We have 3 'sections' of metadata: 'user', 'bucket', and 'bucket.instance'. +You can use the following commands to introspect metadata entries: :: + + $ radosgw-admin metadata list + $ radosgw-admin metadata list bucket + $ radosgw-admin metadata list bucket.instance + $ radosgw-admin metadata list user + + $ radosgw-admin metadata get bucket:<bucket> + $ radosgw-admin metadata get bucket.instance:<bucket>:<bucket_id> + $ radosgw-admin metadata get user:<user> # get or set + +Some variables have been used in above commands, they are: + +- user: Holds user information +- bucket: Holds a mapping between bucket name and bucket instance id +- bucket.instance: Holds bucket instance information[2] + +Every metadata entry is kept on a single rados object. +See below for implementation defails. + +Note that the metadata is not indexed. When listing a metadata section we do a +rados pgls operation on the containing pool. + +Bucket Index +^^^^^^^^^^^^ + +It's a different kind of metadata, and kept separately. The bucket index holds +a key-value map in rados objects. By default it is a single rados object per +bucket, but it is possible since Hammer to shard that map over multiple rados +objects. The map itself is kept in omap, associated with each rados object. +The key of each omap is the name of the objects, and the value holds some basic +metadata of that object -- metadata that shows up when listing the bucket. +Also, each omap holds a header, and we keep some bucket accounting metadata +in that header (number of objects, total size, etc.). + +Note that we also hold other information in the bucket index, and it's kept in +other key namespaces. We can hold the bucket index log there, and for versioned +objects there is more information that we keep on other keys. + +Data +^^^^ + +Objects data is kept in one or more rados objects for each rgw object. + +Object Lookup Path +------------------ + +When accessing objects, ReST APIs come to RGW with three parameters: +account information (access key in S3 or account name in Swift), +bucket or container name, and object name (or key). At present, RGW only +uses account information to find out the user ID and for access control. +Only the bucket name and object key are used to address the object in a pool. + +The user ID in RGW is a string, typically the actual user name from the user +credentials and not a hashed or mapped identifier. + +When accessing a user's data, the user record is loaded from an object +"<user_id>" in pool "default.rgw.meta" with namespace "users.uid". + +Bucket names are represented in the pool "default.rgw.meta" with namespace +"root". Bucket record is +loaded in order to obtain so-called marker, which serves as a bucket ID. + +The object is located in pool "default.rgw.buckets.data". +Object name is "<marker>_<key>", +for example "default.7593.4_image.png", where the marker is "default.7593.4" +and the key is "image.png". Since these concatenated names are not parsed, +only passed down to RADOS, the choice of the separator is not important and +causes no ambiguity. For the same reason, slashes are permitted in object +names (keys). + +It is also possible to create multiple data pools and make it so that +different users buckets will be created in different rados pools by default, +thus providing the necessary scaling. The layout and naming of these pools +is controlled by a 'policy' setting.[3] + +An RGW object may consist of several RADOS objects, the first of which +is the head that contains the metadata, such as manifest, ACLs, content type, +ETag, and user-defined metadata. The metadata is stored in xattrs. +The head may also contain up to 512 kilobytes of object data, for efficiency +and atomicity. The manifest describes how each object is laid out in RADOS +objects. + +Bucket and Object Listing +------------------------- + +Buckets that belong to a given user are listed in an omap of an object named +"<user_id>.buckets" (for example, "foo.buckets") in pool "default.rgw.meta" +with namespace "users.uid". +These objects are accessed when listing buckets, when updating bucket +contents, and updating and retrieving bucket statistics (e.g. for quota). + +See the user-visible, encoded class 'cls_user_bucket_entry' and its +nested class 'cls_user_bucket' for the values of these omap entires. + +These listings are kept consistent with buckets in pool ".rgw". + +Objects that belong to a given bucket are listed in a bucket index, +as discussed in sub-section 'Bucket Index' above. The default naming +for index objects is ".dir.<marker>" in pool "default.rgw.buckets.index". + +Footnotes +--------- + +[1] Omap is a key-value store, associated with an object, in a way similar +to how Extended Attributes associate with a POSIX file. An object's omap +is not physically located in the object's storage, but its precise +implementation is invisible and immaterial to RADOS Gateway. +In Hammer, one LevelDB is used to store omap in each OSD. + +[2] Before the Dumpling release, the 'bucket.instance' metadata did not +exist and the 'bucket' metadata contained its information. It is possible +to encounter such buckets in old installations. + +[3] The pool names have been changed starting with the Infernalis release. +If you are looking at an older setup, some details may be different. In +particular there was a different pool for each of the namespaces that are +now being used inside the default.root.meta pool. + +Appendix: Compendium +-------------------- + +Known pools: + +.rgw.root + Unspecified region, zone, and global information records, one per object. + +<zone>.rgw.control + notify.<N> + +<zone>.rgw.meta + Multiple namespaces with different kinds of metadata: + + namespace: root + <bucket> + .bucket.meta.<bucket>:<marker> # see put_bucket_instance_info() + + The tenant is used to disambiguate buckets, but not bucket instances. + Example:: + + .bucket.meta.prodtx:test%25star:default.84099.6 + .bucket.meta.testcont:default.4126.1 + .bucket.meta.prodtx:testcont:default.84099.4 + prodtx/testcont + prodtx/test%25star + testcont + + namespace: users.uid + Contains _both_ per-user information (RGWUserInfo) in "<user>" objects + and per-user lists of buckets in omaps of "<user>.buckets" objects. + The "<user>" may contain the tenant if non-empty, for example:: + + prodtx$prodt + test2.buckets + prodtx$prodt.buckets + test2 + + namespace: users.email + Unimportant + + namespace: users.keys + 47UA98JSTJZ9YAN3OS3O + + This allows radosgw to look up users by their access keys during authentication. + + namespace: users.swift + test:tester + +<zone>.rgw.buckets.index + Objects are named ".dir.<marker>", each contains a bucket index. + If the index is sharded, each shard appends the shard index after + the marker. + +<zone>.rgw.buckets.data + default.7593.4__shadow_.488urDFerTYXavx4yAd-Op8mxehnvTI_1 + <marker>_<key> + +An example of a marker would be "default.16004.1" or "default.7593.4". +The current format is "<zone>.<instance_id>.<bucket_id>". But once +generated, a marker is not parsed again, so its format may change +freely in the future. |