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

Using swift

The quickest way to try out this feature is use the swift Swift Tool included with the python-swiftclient library. You can use the -S option to specify the segment size to use when splitting a large file. For example:

swift upload test_container -S 1073741824 large_file

This would split the large_file into 1G segments and begin uploading those segments in parallel. Once all the segments have been uploaded, swift will then create the manifest file so the segments can be downloaded as one.

So now, the following swift command would download the entire large object:

swift download test_container large_file

swift uses a strict convention for its segmented object support. In the above example it will upload all the segments into a second container named test_container_segments. These segments will have names like large_file/1290206778.25/21474836480/00000000, large_file/1290206778.25/21474836480/00000001, etc.

The main benefit for using a separate container is that the main container listings will not be polluted with all the segment names. The reason for using the segment name format of <name>/<timestamp>/<size>/<segment> is so that an upload of a new file with the same name won’t overwrite the contents of the first until the last moment when the manifest file is updated.

swift will manage these segment files for you, deleting old segments on deletes and overwrites, etc. You can override this behavior with the --leave-segments option if desired; this is useful if you want to have multiple versions of the same large object available.

Direct API

You can also work with the segments and manifests directly with HTTP requests instead of having swift do that for you. You can just upload the segments like you would any other object and the manifest is just a zero-byte file with an extra X-Object-Manifest header.

All the object segments need to be in the same container, have a common object name prefix, and their names sort in the order they should be concatenated. They don’t have to be in the same container as the manifest file will be, which is useful to keep container listings clean as explained above with swift.

The manifest file is simply a zero-byte file with the extra X-Object-Manifest: <container>/<prefix> header, where <container> is the container the object segments are in and <prefix> is the common prefix for all the segments.

It is best to upload all the segments first and then create or update the manifest. In this way, the full object won’t be available for downloading until the upload is complete. Also, you can upload a new set of segments to a second location and then update the manifest to point to this new location. During the upload of the new segments, the original manifest will still be available to download the first set of segments.

Here’s an example using curl with tiny 1-byte segments:

# First, upload the segments
curl -X PUT -H 'X-Auth-Token: <token>' \
    http://<storage_url>/container/myobject/1 --data-binary '1'
curl -X PUT -H 'X-Auth-Token: <token>' \
    http://<storage_url>/container/myobject/2 --data-binary '2'
curl -X PUT -H 'X-Auth-Token: <token>' \
    http://<storage_url>/container/myobject/3 --data-binary '3'

# Next, create the manifest file
curl -X PUT -H 'X-Auth-Token: <token>' \
    -H 'X-Object-Manifest: container/myobject/' \
    http://<storage_url>/container/myobject --data-binary ''

# And now we can download the segments as a single object
curl -H 'X-Auth-Token: <token>' \
    http://<storage_url>/container/myobject

Static Large Objects

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, except the last, must be above 1 MB (by default) in size. Please see the SLO docs for Static Large Objects 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.
  • 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.

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