An Incremental File System Consistency Checker for Block-Level CDP Systems

A block-level continuous data protection (CDP) system logs every disk block update from an application server (e.g., a file or DBMS server) to a storage system so that any disk updates within a time window are undoable, and thus is able to provide a more flexible and efficient data protection service than conventional periodic data backup systems. Unfortunately, no existing block-level CDP systems can support arbitrary point-in-time snapshots that are guaranteed to be consistent with respect to the metadata of the application server. This deficiency seriously limits the flexibility in recovery point objective (RTO) of block-level CDP systems from the standpoint of the application servers whose data they protect. This paper describes an incremental file system check mechanism (iFSCK) that is designed to address this deficiency for file servers, and exploits file system-specific knowledge to quickly fix an arbitrary point-in-time block-level snapshot so that it is consistent with respect to file system metadata. Performance measurements taken from a fully operational iFSCK prototype show that iFSCK can turn a 10 GB point-in-time block-level snapshot to be file-system consistent in less than 1 second, and takes less than 25% of the time required by the Fsck utility for vanilla ext3 under relaxed metadata consistency requirements.