The Account Reaper

The Account Reaper

The Account Reaper removes data from deleted accounts in the background.

An account is marked for deletion by a reseller issuing a DELETE request on the account’s storage URL. This simply puts the value DELETED into the status column of the account_stat table in the account database (and replicas), indicating the data for the account should be deleted later.

There is normally no set retention time and no undelete; it is assumed the reseller will implement such features and only call DELETE on the account once it is truly desired the account’s data be removed. However, in order to protect the Swift cluster accounts from an improper or mistaken delete request, you can set a delay_reaping value in the [account-reaper] section of the account-server.conf to delay the actual deletion of data. At this time, there is no utility to undelete an account; one would have to update the account database replicas directly, setting the status column to an empty string and updating the put_timestamp to be greater than the delete_timestamp. (On the TODO list is writing a utility to perform this task, preferably through a REST call.)

The account reaper runs on each account server and scans the server occasionally for account databases marked for deletion. It will only trigger on accounts that server is the primary node for, so that multiple account servers aren’t all trying to do the same work at the same time. Using multiple servers to delete one account might improve deletion speed, but requires coordination so they aren’t duplicating effort. Speed really isn’t as much of a concern with data deletion and large accounts aren’t deleted that often.

The deletion process for an account itself is pretty straightforward. For each container in the account, each object is deleted and then the container is deleted. Any deletion requests that fail won’t stop the overall process, but will cause the overall process to fail eventually (for example, if an object delete times out, the container won’t be able to be deleted later and therefore the account won’t be deleted either). The overall process continues even on a failure so that it doesn’t get hung up reclaiming cluster space because of one troublesome spot. The account reaper will keep trying to delete an account until it eventually becomes empty, at which point the database reclaim process within the db_replicator will eventually remove the database files.

Sometimes a persistent error state can prevent some object or container from being deleted. If this happens, you will see a message such as “Account <name> has not been reaped since <date>” in the log. You can control when this is logged with the reap_warn_after value in the [account-reaper] section of the account-server.conf file. By default this is 30 days.

History

At first, a simple approach of deleting an account through completely external calls was considered as it required no changes to the system. All data would simply be deleted in the same way the actual user would, through the public REST API. However, the downside was that it would use proxy resources and log everything when it didn’t really need to. Also, it would likely need a dedicated server or two, just for issuing the delete requests.

A completely bottom-up approach was also considered, where the object and container servers would occasionally scan the data they held and check if the account was deleted, removing the data if so. The upside was the speed of reclamation with no impact on the proxies or logging, but the downside was that nearly 100% of the scanning would result in no action creating a lot of I/O load for no reason.

A more container server centric approach was also considered, where the account server would mark all the containers for deletion and the container servers would delete the objects in each container and then themselves. This has the benefit of still speedy reclamation for accounts with a lot of containers, but has the downside of a pretty big load spike. The process could be slowed down to alleviate the load spike possibility, but then the benefit of speedy reclamation is lost and what’s left is just a more complex process. Also, scanning all the containers for those marked for deletion when the majority wouldn’t be seemed wasteful. The db_replicator could do this work while performing its replication scan, but it would have to spawn and track deletion processes which seemed needlessly complex.

In the end, an account server centric approach seemed best, as described above.

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