Document backups for homedirs on AWS

[consideRatio]

With #3182 we have backups enabled on all the homedir storage on AWS. I was initially worried about costs, but reading up about it I'm less concerned now. This is anyhow something that we should probably end up documenting, so I've summarized some learnings below.

Related info

• Default retention policy (used by us)

  The default EFS backup plan uses the AWS Backup recommended settings for automatic backups—daily backups with a 35-day retention period.

• Backup consistency

  To ensure consistent backups, we recommend that you pause applications or processes that are modifying the file system for the duration of the backup process. Or, schedule your backups to occur during periods when the file system is not being modified.

• Backup performance

  In general, you can expect the following backup rates with AWS Backup:

  • 100 MB/s for file systems composed of mostly large files
  • 500 files/s for file systems composed of mostly small files
    The maximum duration for a backup operation in AWS Backup is thirty days.

• Incremental backups

  AWS Backup performs incremental backups of EFS file systems. During the initial backup, a copy of the entire file system is made. During subsequent backups of that file system, only files and directories that have been changed, added, or removed are copied. With each incremental backup, AWS Backup retains the necessary reference data to allow a full restore. This approach minimizes the time required to complete the backup and saves on storage costs by not duplicating data.

• Complete and partial restores

  You can perform a Complete restore, which restores the entire file system. Or, you can restore specific files and directories using a Partial restore.

[damianavila]

Additionally, I think we might be interested in documenting a "recovery" scenario where some information from homesdirs is somehow lost.