@InProceedings{patterson:snapmirror,
  author = {{R. Hugo} Patterson and Stephen Manley and Mike Federwisch and Dave
  Hitz and Steve Kleiman and Shane Owara},
  title = {{SnapMirror}: File-System-Based Asynchronous Mirroring for Disaster
  Recovery},
  booktitle = {Proceedings of the USENIX FAST '02 Conference on File and
  Storage Technologies},
  year = {2002},
  month = {January},
  pages = {117--130},
  publisher = {USENIX Association},
  address = {Monterey, CA},
  URL =
  {http://www.usenix.org/publications/library/proceedings/fast02/patterson.html},
  keywords = {file systems, pario-bib},
  abstract = {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{PCT} to 80{PCT}. 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{PCT}. dollars depending on the size of the enterprise and the
  role of the data. With increasing frequency, companies are instituting
  disaster recovery plans to ensure appropriate data availability in the event
  of a catastrophic failure or disaster that destroys a site (e.g. flood, fire,
  or earthquake). It is relatively easy to provide redundant server and storage
  hardware to protect against the loss of physical resources. Without the data,
  however, the redundant hardware is of little use.}
}