06ff865d19
Currently when using fast-post, the manifest is updated with the given 'x-object-manifest' header on a POST. If no such header is supplied, then the manifest will change to a regular object. This is not currently true when using post-as-copy. This patch changes the DLO POST using post-as-copy behavior to match that of using fast-post. It was also documented that 'x-object-manifest' must be provided on a POST to a manifest file. Change-Id: Ie1143ab1a2c8f8c21e258a36badbff5d947769d4 Closes-bug: 1612991
157 lines
6.7 KiB
ReStructuredText
157 lines
6.7 KiB
ReStructuredText
.. _large-objects:
|
|
|
|
====================
|
|
Large Object Support
|
|
====================
|
|
|
|
--------
|
|
Overview
|
|
--------
|
|
|
|
Swift has a limit on the size of a single uploaded object; by default this is
|
|
5GB. However, the download size of a single object is virtually unlimited with
|
|
the concept of segmentation. Segments of the larger object are uploaded and a
|
|
special manifest file is created that, when downloaded, sends all the segments
|
|
concatenated as a single object. This also offers much greater upload speed
|
|
with the possibility of parallel uploads of the segments.
|
|
|
|
.. _dynamic-large-objects:
|
|
|
|
.. _dlo-doc:
|
|
|
|
---------------------
|
|
Dynamic Large Objects
|
|
---------------------
|
|
|
|
.. automodule:: swift.common.middleware.dlo
|
|
:members:
|
|
:show-inheritance:
|
|
|
|
.. _static-large-objects:
|
|
|
|
.. _slo-doc:
|
|
|
|
--------------------
|
|
Static Large Objects
|
|
--------------------
|
|
|
|
.. automodule:: swift.common.middleware.slo
|
|
:members:
|
|
:show-inheritance:
|
|
|
|
----------
|
|
Direct API
|
|
----------
|
|
|
|
SLO support centers around the user generated manifest file. After the user
|
|
has uploaded the segments into their account a manifest file needs to be
|
|
built and uploaded. All object segments, must be at least 1 byte
|
|
in size. Please see the SLO docs for :ref:`slo-doc` further
|
|
details.
|
|
|
|
----------------
|
|
Additional Notes
|
|
----------------
|
|
|
|
* With a ``GET`` or ``HEAD`` of a manifest file, the ``X-Object-Manifest:
|
|
<container>/<prefix>`` header will be returned with the concatenated object
|
|
so you can tell where it's getting its segments from.
|
|
|
|
* When updating a manifest object using a POST request, a
|
|
``X-Object-Manifest`` header must be included for the object to
|
|
continue to behave as a manifest object.
|
|
|
|
* The response's ``Content-Length`` for a ``GET`` or ``HEAD`` on the manifest
|
|
file will be the sum of all the segments in the ``<container>/<prefix>``
|
|
listing, dynamically. So, uploading additional segments after the manifest is
|
|
created will cause the concatenated object to be that much larger; there's no
|
|
need to recreate the manifest file.
|
|
|
|
* The response's ``Content-Type`` for a ``GET`` or ``HEAD`` on the manifest
|
|
will be the same as the ``Content-Type`` set during the ``PUT`` request that
|
|
created the manifest. You can easily change the ``Content-Type`` by reissuing
|
|
the ``PUT``.
|
|
|
|
* The response's ``ETag`` for a ``GET`` or ``HEAD`` on the manifest file will
|
|
be the MD5 sum of the concatenated string of ETags for each of the segments
|
|
in the manifest (for DLO, from the listing ``<container>/<prefix>``).
|
|
Usually in Swift the ETag is the MD5 sum of the contents of the object, and
|
|
that holds true for each segment independently. But it's not meaningful to
|
|
generate such an ETag for the manifest itself so this method was chosen to
|
|
at least offer change detection.
|
|
|
|
|
|
.. note::
|
|
|
|
If you are using the container sync feature you will need to ensure both
|
|
your manifest file and your segment files are synced if they happen to be
|
|
in different containers.
|
|
|
|
-------
|
|
History
|
|
-------
|
|
|
|
Dynamic large object support has gone through various iterations before
|
|
settling on this implementation.
|
|
|
|
The primary factor driving the limitation of object size in Swift is
|
|
maintaining balance among the partitions of the ring. To maintain an even
|
|
dispersion of disk usage throughout the cluster the obvious storage pattern
|
|
was to simply split larger objects into smaller segments, which could then be
|
|
glued together during a read.
|
|
|
|
Before the introduction of large object support some applications were already
|
|
splitting their uploads into segments and re-assembling them on the client
|
|
side after retrieving the individual pieces. This design allowed the client
|
|
to support backup and archiving of large data sets, but was also frequently
|
|
employed to improve performance or reduce errors due to network interruption.
|
|
The major disadvantage of this method is that knowledge of the original
|
|
partitioning scheme is required to properly reassemble the object, which is
|
|
not practical for some use cases, such as CDN origination.
|
|
|
|
In order to eliminate any barrier to entry for clients wanting to store
|
|
objects larger than 5GB, initially we also prototyped fully transparent
|
|
support for large object uploads. A fully transparent implementation would
|
|
support a larger max size by automatically splitting objects into segments
|
|
during upload within the proxy without any changes to the client API. All
|
|
segments were completely hidden from the client API.
|
|
|
|
This solution introduced a number of challenging failure conditions into the
|
|
cluster, wouldn't provide the client with any option to do parallel uploads,
|
|
and had no basis for a resume feature. The transparent implementation was
|
|
deemed just too complex for the benefit.
|
|
|
|
The current "user manifest" design was chosen in order to provide a
|
|
transparent download of large objects to the client and still provide the
|
|
uploading client a clean API to support segmented uploads.
|
|
|
|
To meet an many use cases as possible Swift supports two types of large
|
|
object manifests. Dynamic and static large object manifests both support
|
|
the same idea of allowing the user to upload many segments to be later
|
|
downloaded as a single file.
|
|
|
|
Dynamic large objects rely on a container listing to provide the manifest.
|
|
This has the advantage of allowing the user to add/removes segments from the
|
|
manifest at any time. It has the disadvantage of relying on eventually
|
|
consistent container listings. All three copies of the container dbs must
|
|
be updated for a complete list to be guaranteed. Also, all segments must
|
|
be in a single container, which can limit concurrent upload speed.
|
|
|
|
Static large objects rely on a user provided manifest file. A user can
|
|
upload objects into multiple containers and then reference those objects
|
|
(segments) in a self generated manifest file. Future GETs to that file will
|
|
download the concatenation of the specified segments. This has the advantage of
|
|
being able to immediately download the complete object once the manifest has
|
|
been successfully PUT. Being able to upload segments into separate containers
|
|
also improves concurrent upload speed. It has the disadvantage that the
|
|
manifest is finalized once PUT. Any changes to it means it has to be replaced.
|
|
|
|
Between these two methods the user has great flexibility in how (s)he chooses
|
|
to upload and retrieve large objects to Swift. Swift does not, however, stop
|
|
the user from harming themselves. In both cases the segments are deletable by
|
|
the user at any time. If a segment was deleted by mistake, a dynamic large
|
|
object, having no way of knowing it was ever there, would happily ignore the
|
|
deleted file and the user will get an incomplete file. A static large object
|
|
would, when failing to retrieve the object specified in the manifest, drop the
|
|
connection and the user would receive partial results.
|