Computerized data has become critical to the survival of an enterprise. Companies must have a strategy for recovering their data should a disaster such as a fire destroy the primary data center. Current mechanisms offer data managers a stark choice: rely on affordable tape but risk the loss of a full day of data and face many hours or even days to recover, or have the benefits of a fully synchronized on-line remote mirror, but pay steep costs in both write latency and network bandwidth to maintain the mirror. In this paper, we argue that asynchronous mirroring, in which batches of updates are periodically sent to the remote mirror, can let data managers find a balance between these extremes. First, by eliminating the write latency issue, asynchrony greatly reduces the performance cost of a remote mirror. Second, by storing up batches of writes, asynchronous mirroring can avoid sending deleted or overwritten data and thereby reduce network bandwidth requirements. Data managers can tune the update frequency to trade network bandwidth against the potential loss of more data. We present Snap-Mirror, an asynchronous mirroring technology that leverages file system snapshots to ensure the consistency of the remote mirror and optimize data transfer. We use traces of production filers to show that even updating an asynchronous mirror every 15 minutes can reduce data transferred by 30% to 80%. We find that exploiting file system knowledge of deletions is critical to achieving any reduction for no-overwrite file systems such as WAFL and LFS. Experiments on a running system show that using file system metadata can reduce the time to identify changed blocks from minutes to seconds compared to purely logical approaches. Finally, we show that using SnapMirror to update every 30 minutes increases the response time of a heavily loaded system only 22%.

As file systems grow in size, ensuring that data is safely stored becomes more and more difficult. Historically, file system backup strategies have focused on logical backup where files are written in their entirety to the backup media. An alternative is physical backup where the disk blocks that make up the file system are written to the backup media. This paper compares logical and physical backup strategies in large file systems. We discuss the advantages and disadvantages of the two approaches, and conclude by showing that while both can achieve good performance, physical backup and restore can achieve much higher throughput while consuming less CPU. In addition, physical backup and restore is much more capable of scaling its performance as more devices are added to a system.

Sharing network data between NT and UNIX systems is becoming increasingly important as NT moves into areas previously serviced entirely by UNIX. One difficulty in sharing data is that the two filesystem security models are quite different. NT file servers use access control lists (ACLs) that allow permissions to be specified for an arbitrary number of users and groups, while UNIX NFS servers use traditional UNIX permissions that provide control only for owner, group, and other. This paper describes an integrated security model in which a single filesystem can contain both files with NT-style ACLs and files with UNIX-style permissions. For native file service requests (NT requests to NT-style files and NFS requests to UNIX-style files) the security model exactly matches an NT or UNIX file-server. For non-native requests, heuristics allow a reasonable level of access without compromising the security guarantees of the native model.

Sharing network data between UNIX and NT systems is becoming increasingly important as NT moves into areas previously serviced entirely by UNIX. One difficulty in sharing data between UNIX and NT is that their file system security models are quite different. NT file servers use access control lists (ACLs) that allow permissions to be specified for an arbitrary number of users and groups, while UNIX NFS servers use traditional UNIX permissions that provide control only for owner, group, and other. This paper describes a merged model in which a single file system can contain both files with NT-style ACLs and files with UNIX-style permissions. For native file service requests (NFS requests to UNIX-style files and NT requests to NT-style files) the security model exactly matches a UNIX or NT fileserver. For non-native requests, heuristics allow a reasonable level of access without compromising the security guarantees of the native model.