This question might look like an easy/idiot/beginner question but I'm really confused between both of them.
Why do I need to use Amazon S3 Glacier if I can use the normal S3 Bucket and just change the storage class of the objects inside to Glacier manually or by using Lifecycle rule?
Thanks in advance,
In the old days, Amazon Glacier was only available as a separate product. Frankly, the Glacier service is a pain to use.
Every request has to be submitted as a Job, which takes a long time to return. Even obtaining a list of archives is slow, let alone restoring a file from the archive.
The best way to use the Amazon Glacier service is with a third-party tool (eg Cloudberry Backup) that knows how to interface with Glacier, isolating you from having to use it directly.
Then, in 2012, the Amazon S3 team introduced a new Glacier Storage Class where S3 would move the data to Glacier, but still present the objects as being "in S3". (Well, the objects appear in S3 and their metadata is accessible, but the contents of the objects is stored in Glacier.) Then, in 2019, an even lower-cost Glacier Deep Archive storage class offered even lower prices than available through Amazon Glacier itself.
Therefore, it is now both easier and lower cost to use Glacier via Amazon S3 storage classes.
Amazon Glacier still remains available for use, and has been renamed Amazon S3 Glacier to further confuse things. There might be some use-cases where it is preferable to use (eg acting like traditional tape backups for AWS Storage Gateway Tape Gateways), but Glacier Deep Archive in S3 would be the lowest-cost option.