Patent ID: 7389394

Claim:
A system comprising: a volume server; a first and a second host computer system; and a plurality of physical block devices; wherein the volume server is configured to: aggregate storage in the plurality of physical block devices into a plurality of logical volumes, wherein at least a given one of the plurality of logical volumes includes storage from at least two physical block devices, wherein the volume server is further configured to present the given logical volume to a volume client on one or more of the host computer systems as a virtualized logical block device comprising a plurality of logical block devices hierarchically configured to implement a plurality of virtualization functions, wherein the virtualization functions are implemented by the given logical volume transparently to the volume client; make a first subset of the plurality of logical volumes available to said first host computer system for input/output; and make a second subset of the plurality of logical volumes available to said second host computer system for input/output; wherein said first subset includes at least a particular one of the plurality of logical volumes, wherein the particular logical volume is not included in said second subset; wherein the given logical volume is included in one or more of the first or second subsets; wherein a second one of the plurality of logical volumes is configured to store a copy-on-write snapshot of data stored in the given logical volume, wherein for a particular data block of said given logical volume, said copy-on-write snapshot stored by said second logical volume comprises a copy of said particular data block prior to a write operation directed to said particular data block, wherein said copy of said particular data block of said given logical volume is stored to said second logical volume in response to detecting said write operation directed to said particular data block and before said particular data block is modified on said given logical volume; wherein a fourth one of the plurality of logical volumes is configured to store a copy-on-write snapshot of data stored in a third one of the plurality of logical volumes, wherein for a given data block of said third logical volume, said copy-on-write snapshot stored by said fourth logical volume comprises a copy of said given data block prior to a write operation directed to said given data block, wherein said copy of said given data block is stored to said fourth logical volume in response to detecting said write operation directed to said given data block and before said given data block is modified on said third logical volume; and wherein said fourth and said second logical volumes are configured to store at least a portion of respective snapshot data of said third and said given logical volumes in a shared logical or physical block storage device common to both said fourth and said second logical volumes.