Another common (and valid) complaint regarding PVFS1 is its lack of support for redundancy at the server level. RAID approaches are usable to provide tolerance of disk failures, but if a server disappears, all files with data on that server are inaccessible until the server is recovered.
Traditional high-availability solutions may be applied to both metadata and data servers in PVFS2 (they're actually the same server). This option requires shared storage between the two machines on which file system data is stored, so this may be prohibitively expensive for some users.
A second option that is being investigated is what we are calling lazy redundancy. The lazy redundancy approach is our response to the failure of RAID-like approaches to scale well for large parallel I/O systems when applied across servers. The failure of this approach at this scale is primarily due to the significant change in environment (latency and number of devices across which data is striped). Providing the atomic read/modify/write capability necessary to implement RAID-like protocols in a distributed file system requires a great deal of performance-hampering infrastructure.
With lazy redundancy we expose the creation of redundant data as an explicit operation. This places the burden of enforcing consistent access on the user. However, it also opens up the opportunity for a number of optimizations, such as creating redundant data in parallel. Further, because this can be applied at a more coarse grain, more compute-intensive algorithms may be used in place of simple parity, providing higher reliability than simple parity schemes.
Lazy redundancy is still at the conceptual stage. We're still trying to determine how to best fit this into the system as a whole. However, traditional failover solutions may be put in place for the existing system.