docs(faq): Add more information about using S3 Glacier with rustic

Add additional details on how to use S3 Glacier with rustic, including instructions for storing and retrieving data, and removing cold data. This information will help users configure their repositories to work with S3 Glacier.

Fixes #67

Signed-off-by: simonsan <[email protected]>

src/FAQ.md

@@ -29,7 +29,8 @@ of is that you don't run prune with restic and rustic at the same time.
 - New features (e.g. hot/cold repositories, lock-free pruning)
 - Some commands or options act a bit different or have slightly different syntax
 
-A more detailed comparison can be found in the [comparison table](./comparison-restic.md).
+A more detailed comparison can be found in the
+[comparison table](./comparison-restic.md).
 
 ## Why is rustic written in Rust
 
@@ -52,6 +53,56 @@ your local disc and a second level cache with the "hot repo". Note that the "hot
 repo" can be also a remote repo, so hot/cold repositories also work for multiple
 rustic clients backing up to the same repository.
 
+### More details
+
+rustic doesn't support single repositories/buckets which have objects that can
+be in different "states" (something like this is not contained in the restic
+REST protocol; rclone therefore isn't able to provide this kind of information).
+
+So, to use S3 glacier, you should use 2 buckets: A hot one which is always
+accessible and a cold one where all object should be transitioned/transferred
+directly to Glacier when they are stored. The "hot" objects are then in fact
+stored in both buckets, but this shouldn't be much overhead as it is only
+metadata which usually is less than 1% of the repo size. (The side effect is
+that the cold repo contains a full restic-compatible repository; if you warm up
+it completely, you can use it a standard repository within rustic and even
+restic).
+
+TL;DR: I think you have to do the following:
+
+1. To store data (e.g. `init` or `backup` command):
+
+- Create a regular bucket and use it as `repo-hot`
+- Create a bucket for `cold` and configure it to store data directly in Glacier
+  (don't know if you can configure this in `rclone` or in AWS)
+
+2. To retrieve data (e.g. `restore`):
+
+- configure `warm-up-command` such that objects from the cold buckets are
+  restored, see e.g
+  <https://docs.aws.amazon.com/AmazonS3/latest/userguide/restoring-objects.html>.
+  That is, the config file should look have a line like
+  `warm-up-command  = 'aws s3api restore-object --bucket BUCKET --key path/data/%id --restore-request '{"Days":25,"GlacierJobParameters":{"Tier":"Standard"}}'  '`
+  (may need some adaption)
+- configure a suitable `warm-up-wait` to ensure that the data is warmed up and
+  can be accessed after that
+- Note: All commands which need to read cold data now will warm this data up and
+  wait for the given time before processing the data
+
+3. removing cold data (`prune`) This actually should work out of the box, but
+   note that by default `prune` doesn't repack any cold pack file; instead it
+   keeps them until the last blob within is no longer used.
+
+- to do more repacking, have a look at the `repack-cacheable` option which does
+  allow to repack cold pack files.
+- Use the `keep-pack` option if you have some minimum holding duration, i.e. if
+  you would get charged if you remove objects too early. This option ensures
+  that only pack files older than the given time will be removed.
+
+Once you get a working configuration, please share it
+[in the main repository](https://github.com/rustic-rs/rustic/tree/main/config/services)
+so other users can use it as well!
+
 ## Are all operations lock free?
 
 Yes, all operations are designed lock-free. This means all commands can run