Multipart upload is required when the object size you are uploading is greater than 5 GBs.
As stated in the AWS documentation for S3:
Depending on the size of the data you are uploading, Amazon S3 offers the following options:
Upload objects in a single operation—With a single PUT operation, you can upload objects up to 5 GB in size.
Upload objects in parts—Using the multipart upload API, you can upload large objects, up to 5 TB.
Example:
Load the first part:
$ aws glacier initiate-multipart-upload --account-id - --part-size 1048576 --vault-name my-vault --archive-description "multipart upload test"
This command outputs an upload ID when successful. Use the upload ID when uploading each part of your archive with aws glacier upload-multipart-part as shown next:
Load the rest assuming the returned upload ID is 19gaRezEXAMPLES6Ry5YYdqthHOC_kGRCT03L9yetr220UmPtBYKk-OssZtLqyFu7sY1_lR7vgFuJV6NtcV5zpsJ
(repeat as many times as necessary to consume the object):
aws glacier upload-multipart-part --body saw-09-21-19-part1.7z --range 'bytes 0-1048575/*' --account-id - --vault-name my-vault --upload-id 19gaRezEXAMPLES6Ry5YYdqthHOC_kGRCT03L9yetr220UmPtBYKk-OssZtLqyFu7sY1_lR7vgFuJV6NtcV5zpsJ
Here is step-by-step information on how to do it with the CLI:
https://docs.aws.amazon.com/cli/latest/userguide/cli-services-glacier.html
See here for even more information:
https://docs.aws.amazon.com/AmazonS3/latest/dev/UploadingObjects.html
https://docs.aws.amazon.com/cli/latest/reference/glacier/upload-multipart-part.html