[LU-13032] Add lctl cleanup|uncache|revalidate commands for PCC

• Type: Technical task
• Resolution: Unresolved
• Priority: Minor
• Assignee: Qian Yingjin
• Reporter: Qian Yingjin

Description

If there is a "kept" file on PCC, but the file was modified in Lustre after data restore, then we need to ensure that the stale PCC copy is removed from cache.

Usually there is a daemon running on the PCC client, monitoring the space usage of the PCC device, scanning the PCC device, doing some actions accordingly, which can be used to remove this kind of PCC copies.
We could add some lctl pcc commands or llapi interface as follows:

1. lctl pcc clean $MNTPT $PCCPATH
   The command above can be used to clean up the stale invalid PCC copies out from PCC to free up space.
2. lctl pcc uncache $MNTPT $PCCPATH
   This command will restore all data back to Lustre OSTs, and then remove the PCC copies, similar with lctl pcc del, but does not delete the PCC backend from the client.
3. lctl pcc revalidate $MNTPT $PCCPATH
   This command will try to attach the PCC copies again if it is still valid.
   First, if the Layout generation is consistent, we can attach it directly;
   Otherwise, compare the data version between the value in HSM attrs and the one of the file in Lustre, if they are same, we can also revalidate the PCC cache.