Abstract: The invention relates to host caching in data storage systems. In an embodiment, the invention provides a first host and a second host, each having memory. The host memory includes nonvolatile and volatile portions available for cache. Each host logically owns its volatile memory and the other host's nonvolatile memory. By assigning ownership in this way data can be transmitted between the hosts with little communication overhead. In addition, if the first host fails between write acknowledgment and destaging the write data, the write data is safely stored in the second nonvolatile memory of the second host. Thus, the second host can destage the data from the second nonvolatile memory. In addition, the host cache writes and reads the data rapidly by its nature.

Abstract: The present invention relates to methods and systems of snapshot management of a file system in a data storage system. To represent the snapshots, the invention maintains pointers to the root block pointer of each snapshot. When the active file system is modified, this invention avoids overwriting any blocks used by previous snapshots by allocating new blocks for the modified blocks. When the invention needs to put an established block in a new location, it must update a parent block to point to the new location. The update to the parent block may then require allocating a new block for the new parent block and so forth. Parts of the file system not modified since a snapshot remain in place. The amount of space required to represent snapshots scales with the fraction of the file system that users modify. To maintain snapshot integrity, this invention keeps track of the first and last snapshots that use each block in space map blocks spread throughout the file system data space.