Patent Publication Number: US-2022222017-A1

Title: Global de-duplication of virtual disks in a storage platform

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a Continuation of U.S. patent application Ser. No. 17/028,164 filed on Sep. 22, 2020, which is a Continuation of U.S. patent application Ser. No. 15/155,838 (now U.S. Pat. No. 10,846,024) filed on May 16, 2016, all of which are hereby incorporated by reference in their entireties herein. This application is related to U.S. patent application Ser. Nos. 14/322,813, 14/322,832, 14/684,086, 14/322,850, 14/322,855, 14/322,867, 14/322,868, 14/322871, and 14/723,380 (Attorney Docket Nos. HEDVP001, HEDVP002, HEDVP002X1, HEDVP003, HEDVP004, HEDVP005, HEDVP006, HEDVP007, HEDVP008), which are all hereby incorporated by reference. This application is related to U.S. patent application Ser. No. 15/156,015 (Attorney Docket No. COMMV.496A (was HEDVP013)), which is hereby incorporated by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to data storage within a data center. More specifically, the present invention relates to de-duplication of data on virtual disks over various data centers. 
     BACKGROUND OF THE INVENTION 
     In the field of data storage, enterprises have used a variety of techniques in order to store the data that their software applications use. At one point in time, each individual computer server within an enterprise running a particular software application (such as a database or e-mail application) would store data from that application in any number of attached local disks. Although this technique was relatively straightforward, it led to storage manageability problems in that the data was stored in many different places throughout the enterprise. 
     These problems led to the introduction of the storage area network in which each computer server within an enterprise communicated with a central storage computer node that included all of the storage disks. The application data that used to be stored locally at each computer server was now stored centrally on the central storage node via a fiber channel switch, for example. Although such a storage area network was easier to manage, changes in computer server architecture created new problems. 
     With the advent of virtualization, each computer server can now host dozens of software applications through the use of a hypervisor on each computer server and the use of virtual machines. Thus, computer servers which had been underutilized could now host many different server applications, each application needing to store its data within the storage area network. Weaknesses in the storage area network were revealed by the sheer number of server applications needing to access disks within the central storage node. And, even with the use of remote storage platforms (such as “in-the-cloud” storage), problems still exist. 
     For example, the sheer amount of data that applications desire to store in a remote storage platform can overwhelm the platform, raise costs, and lead to inefficiency. Attempts to remove duplicates of data have been tried but are not optimal. Accordingly, further techniques and systems are desired to remove duplicates of data within a storage platform. 
     SUMMARY OF THE INVENTION 
     To achieve the foregoing, and in accordance with the purpose of the present invention, techniques are disclosed that provide the advantages discussed below. 
     In order to avoid writing duplicates of blocks of data into a storage platform, any virtual disk within the storage platform may have a de-duplication feature enabled. In one embodiment, all virtual disks have this feature enabled. For virtual disks with de-duplication enabled, a unique message digest is calculated for every block of data written to that virtual disk. Upon a write, these message digests may be consulted in order to determine if a particular block of data has already been written, if so, it is not written again, and if not, it is written. In one particular embodiment, all de-duplication virtual disks are written to a single system virtual disk within the storage platform. De-duplication occurs over the entire storage platform and over all of its virtual disks because all message digests are consulted before a write is performed for any virtual disk. 
     In a first embodiment when writing to a virtual disk, the method first calculates a hash value for each block of data to be written to the storage platform from the virtual disk. Each of these hash values is compared against a table that includes all hash values of all data blocks that have been written to the storage platform. If the hash value is present, this means that the data block has already been written to the storage platform, it is not written again, and a system disk offset is retrieved from the table that corresponds to the hash value. The system disk offset is then stored in metadata of the storage platform indicating where the block of data is stored. If the hash value is not present, then the block of data is written to the storage platform into the system virtual disk at a system disk offset. 
     In a second embodiment when the reading from a virtual disk that has de-duplication enabled, a read request from a software application includes a virtual disk offset. Using the virtual disk offset, a virtual machine retrieves from the storage platform a corresponding system disk offset of the system virtual disk. The system virtual disk is used by all virtual disks of the storage platform that have de-duplication enabled and does not include any duplicate blocks of data. Next, the system disk offset is used to read from the system virtual disk from one of the storage nodes of the storage platform, the data desired by the read request. The data is then returned to the requesting software application. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention, together with further advantages thereof, may best be understood by reference to the following description taken in conjunction with the accompanying drawings in which: 
         FIG. 1  illustrates a data storage system having a storage platform according to one embodiment of the invention. 
         FIG. 2  illustrates software modules used within the storage system. 
         FIG. 3  illustrates in greater detail the computer servers in communication with the storage platform. 
         FIG. 4  illustrates a user interface window presented to an administrator in the course of provisioning a virtual disk within the platform. 
         FIG. 5  is a symbolic representation of a virtual disk showing how data within the virtual disk is stored within the storage platform and is referred to in the description of  FIG. 9 . 
         FIG. 6  illustrates a de-duplication table that is maintained by the metadata module of each storage node. 
         FIG. 7  is an example of a system de-duplication disk found within the storage platform. 
         FIG. 8  is a flow diagram describing one embodiment by which a virtual machine writes data to the storage platform that will not be duplicated. 
         FIG. 9  describes one embodiment by which a CVM writes unique blocks to a virtual disk in the storage platform. 
         FIGS. 10 and 11  illustrate metadata storage within the platform. 
         FIG. 12  is a flow diagram describing one embodiment by which a virtual machine reads data from the storage platform. 
         FIG. 13  shows information that may be stored within a memory cache of a controller virtual machine (CVM) on one of the computer servers in the compute farm. 
         FIGS. 14 and 15  illustrate a computer system suitable for implementing embodiments of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Storage System 
       FIG. 1  illustrates a data storage system  10  according to one embodiment of the invention having a storage platform  20 . Included within the storage platform  20  are any number of computer nodes  30 - 40 . Each computer node of the storage platform has a unique identifier (e.g., “A”) that uniquely identifies that computer node within the storage platform. Each computer node is a computer having any number of hard drives and solid-state drives (e.g., flash drives), and in one embodiment includes about twenty disks of about 1 TB each. A typical storage platform may include on the order of about 81 TB and may include any number of computer nodes. One advantage is that a platform may start with as few as three nodes and then grow incrementally to as large as 1,000 nodes or more. 
     Computers nodes  30 - 40  are shown logically being grouped together, although they may be spread across data centers and may be in different geographic locations. A management console  40  used for provisioning virtual disks within the storage platform communicates with the platform over a link  44 . Any number of remotely located computer servers  50 - 52  each typically executes a hypervisor in order to host any number of virtual machines. Server computers  50 - 52  form what is typically referred to as a compute farm, and although such a compute farm may belong to a single company, it is contemplated that any number of such computer servers or compute farms will be storing data within storage platform  20 , and that these computer servers may represent many different companies and enterprises. 
     As shown, these virtual machines may be implementing any of a variety of applications such as a database server, an e-mail server, etc., including applications from companies such as Oracle, Microsoft, etc. These applications write to and read data from the storage platform using a suitable storage protocol such as iSCSI or NFS, although each application will not be aware that data is being transferred over link  54  using a different protocol. 
     Management console  40  is any suitable computer or computers able to communicate over an Internet connection or link  44  with storage platform  20 . When an administrator of a particular enterprise wishes to manage the storage platform (e.g., provisioning a virtual disk, snapshots, revert, clone, analyze metrics, determine health of cluster, etc.) he or she uses the management console to access the storage platform and is put in communication with a management console routine executing as part of metadata module  130  on any one of the computer nodes within the platform. The management console routine is typically a Web server application. 
     In order to provision a new virtual disk within storage platform  20  for a particular application running on a virtual machine, the virtual disk is first created and then attached to a particular virtual machine. In order to create a virtual disk, a user uses the management console to first select the size of the virtual disk (e.g., 100 GB), and then selects the individual policies that will apply to that virtual disk. For example, the user selects a replication factor, a data center aware policy, whether de-duplication is enabled, and other policies concerning whether or not to compress the data, the type of disk storage, etc. Once the virtual disk has been created, it is then attached to a particular virtual machine within one of the computer servers  50 - 52  and the provisioning process is complete. 
     Advantageously, storage platform  20  is able to simulate prior art central storage nodes (such as the VMax and Clarion products from EMC, VMWare products, etc.) and the virtual machines and application servers will be unaware that they are communicating with storage platform  20  instead of a prior art central storage node. In addition, the provisioning process can be completed on the order of minutes or less, rather than in four to eight weeks as was typical with prior art techniques. The advantage is that one only need to add metadata concerning a new virtual disk in order to provision the disk and have the disk ready to perform writes and reads. No allocation of actual physical storage is needed. 
     Thus, storage system  10  may be utilized by any number of companies, clients, enterprises etc., each of whom creates any number virtual disks to be stored within storage platform  20 . The system will store the data pertaining to these various virtual disks in any suitable location, and is then able to perform de-duplication of data at the system level rather than at a more local level. 
       FIG. 2  illustrates software modules used within storage system  10 . Shown is a data module  120 , a metadata module  130  and a pod module  140 . Data module handles the storage of data (such as blocks of data, files, objects, etc.) onto individual computer nodes  30 - 40 . Metadata module handles the storage of metadata within the platform, that is, data that helps describe the data. Metadata includes all of the virtual disk information described below in  FIG. 4 , for example. Pod module is a coordinator module and also stores transaction states. 
     Although shown as three modules, each of the modules runs independently on each of the computer nodes within the platform  20 . Also, associated with each module on each node is a memory cache  122 ,  132  and  142  that stores information used by that module; each module on each computer node may also use persistent storage on that node. A file (for example) that is stored on nodes  32 ,  34  and  36  ( FIG. 1 ), is referred to as being stored on its “data nodes  32 ,  34  and  36 .” The metadata for that file may be stored on three different nodes, and those nodes for that file are referred to as the “metadata nodes  30 ,  36  and  38 .” The data nodes and metadata nodes for a particular stored file may be the same or may be different. The modules communicate with each other via a modified version of Gossip over TCP, and work in concert to manage the storage platform. 
     Controller Virtual Machine 
       FIG. 3  illustrates in greater detail one of the computer servers  51  in communication with storage platform  20 . As mentioned above, each computer server may host any number of virtual machines, each executing a particular software application. The application may perform I/O handling using a block-based protocol such as iSCSI, using a file-based protocol such as NFS, and the virtual machine communicates using this protocol. Of course, other suitable protocols may also be used by an application. The actual communication protocol used between the server and platform is transparent to these virtual machines. 
     As shown, server  51  includes a hypervisor and virtual machines  182  and  186  that desire to perform I/O handling using the iSCSI protocol  187  or the NFS protocol  183 . Server  51  also includes a specialized controller virtual machine (CVM)  180  that is adapted to handle communications with the virtual machines using either protocol (and others), yet communicates with the storage platform using a proprietary protocol  189 . Protocol  189  may be any suitable protocol for passing data between storage platform  20  and a remote computer server  51  such as TCP. In addition, the CVM may also communicate with public cloud storage using the same or different protocol  191 . Advantageously, the CVM need not communicate any “liveness” information between itself and the computer nodes of the platform. There is no need for any CVM to track the status of nodes in the cluster. The CVM need only talk to a node in the platform, which is then able to route requests to other nodes and public storage nodes. 
     The CVM also uses a memory cache  181  on the computer server  51 . In communication with computer server  51  and with CVM  180  are also any number of solid-state disks (or other similar persistent storage)  195 . These disks may be used as a data cache to also store data blocks that are written into storage platform  20 . This cache may be used to rapidly retrieve data blocks instead of retrieving them from the remote storage platform. 
     CVM  180  handles different protocols by simulating an entity that the protocol would expect. For example, when communicating under the iSCSI block protocol, CVM responds to an iSCSI Initiation by behaving as an iSCSI Target. In other words, when virtual machine  186  performs I/O handling, it is the iSCSI Initiator and the controller virtual machine is the iSCSI Target. When an application is using the block protocol, the CVM masquerades as the iSCSI Target, traps the iSCSI CDBs, translates this information into its own protocol, and then communicates this information to the storage platform. Thus, when the CVM presents itself as an iSCSI Target, the application may simply talk to a block device as it would do normally. 
     Similarly, when communicating with an NFS client, the CVM behaves as an NFS server. When virtual machine  182  performs I/O handling the controller virtual machine is the NFS server and the NFS client (on behalf of virtual machine  182 ) executes either in the hypervisor of computer server  51  or in the operating system kernel of virtual machine  182 . Thus, when an application is using the NFS protocol, the CVM masquerades as an NFS server, captures NFS packets, and then communicates this information to the storage platform using its own protocol. 
     An application is unaware that the CVM is trapping and intercepting its calls under the SCSI or NFS protocol, or that the CVM even exists. One advantage is that an application need not be changed in order to write to and read from the storage platform. Use of the CVM allows an application executing upon a virtual machine to continue using the protocol it expects, yet allows these applications on the various computer servers to write data to and read data from the same storage platform  20 . 
     Replicas of a virtual disk may be stored within public cloud storage  190 . As known in the art, public cloud storage refers to those data centers operated by enterprises that allow the public to store data for a fee. Included within these data centers are those known as Amazon Web Services and Google Compute. During a write request, the write request will include an identifier for each computer node to which a replica should be written. For example, nodes may be identified by their IP address. Thus, the computer node within the platform that first fields the write request from the CVM will then route the data to be written to nodes identified by their IP addresses. Any replica that should be sent to the public cloud can then simply be sent to the DNS name of a particular node which request (and data) is then routed to the appropriate public storage cloud. Any suitable computer router within the storage platform may handle this operation. 
     Global De-Duplication of Data 
     As mentioned earlier, prior art techniques are not optimal at removing duplicates of data (i.e. “de-duplication” of data) that may be stored within a storage platform. Prior art techniques typically only attempt to remove duplicates of data at the disk or volume level. Even if a prior art technique were to remove duplicates at a cluster level or data center level, there would still be inefficiency. The inventors of the present invention have realized that as a storage system, such as storage system  10 , serves many different clients, and is able to store data within a variety of data centers and public cloud storage, that removal of duplicate data should occur at the system level, over all clients, data centers and storage platforms encompassed within the storage system. Thus, de-duplication can occur over all virtual disks within the storage system, regardless of which client owns which virtual disk, and regardless of where the virtual disk is physically stored. 
     Moreover, it is realized that de-duplication can occur locally in a compute farm, as data is being written to a storage platform, rather than attempting to iterate over all computers and disk drives within the storage platform after the data has been stored. In addition, such de-duplication still respects the concept of data replication, and can perform de-duplication on any size data block. 
     Provision Virtual Disk for De-Duplication 
       FIG. 4  illustrates a user interface window  220  presented to an administrator in the course of provisioning a virtual disk within platform  20 . In this situation, the administrator is aware that a particular software application desires a virtual disk within the platform and is aware of the characteristics that the virtual disk should have. The administrator first uses the management console to access the platform and connect with the management console Web server on any one of the computer nodes within the platform. One of the characteristics for the virtual disk that may be chosen is whether or not the virtual disk should be de-duplicated. Applications that typically store unique data such as image files may not desire de-duplication (as de-duplication can add overhead), while applications that store text files (such as e-mail messages) may desire de-duplication. De-duplication, thus, is an optional feature that may be turned on or off for each virtual disk. 
     The administrator chooses the “Add Disk” option from within the management console routine and is presented with a window such as window  220  in order to choose the characteristics of the new virtual disk. For example, the administrator chooses: a name  224  for the new virtual disk; a size  226  for the virtual disk; a replication factor  228  (indicating how many replicas of the data should be stored within the platform); a residence  230  (indicating whether the data on the virtual disk should be stored on hard disk drives, on flash drives or on any other type of storage drive); compressed  232  (indicating whether the data on the virtual disk should be compressed or not); a replication policy  236  (agnostic, data center aware, rack aware, or hybrid cloud aware); cache enabled  238  (a quality of service choice); and disk type  240  (indicating whether the virtual disk is of a block type—the iSCSI protocol—or whether the virtual disk is of a file type—the NFS protocol). 
     Option de-duplication  234  indicates whether duplicates of the data should be saved to the virtual disk or not. Selecting “True” indicates that de-duplication is turned “on” and that duplicates of the data for this virtual disk will not be written to the platform, while selecting “False” indicates that de-duplication is turned “off” and that it is possible that duplicates of the data will be written to the platform. 
     Once chosen, these characteristics are stored as “virtual disk information”  260  onto computer nodes within the storage platform. Preferably, this virtual disk information is replicated and stored on at least three different computer nodes. The metadata module  130  on these three computer nodes handles the storage of this information by placing it into persistent storage as shown in  FIG. 10  as described below. In one embodiment, a hash function is used upon the virtual disk name  224  in order to produce a hash value which is then used to select three computer nodes within the platform. For example, the virtual disk information for the newly created virtual disk may be stored upon nodes  36 ,  30  and  40 . In this fashion, the virtual disk metadata has been stored upon three metadata nodes within the platform (which might be different from the nodes where the actual data of the virtual disk will be stored). 
     The virtual disk that has been created is also attached to a virtual machine of the compute farm. In this step, the administrator is aware of which virtual machine on which computer of the compute farm needs the virtual disk. Thus, information regarding the newly created virtual disk (i.e., name, space available, etc.) is sent from the management console routine over link  189  to the appropriate computer within the compute farm. The information is provided to the controller virtual machine  180  which stores the information in cache  181 , ready for use when the virtual machine needs to write or to read data. The administrator also supplies the name of the virtual disk to the application that will use it. At this time, no information need be sent from the storage platform or from the CVM to the application. Cache  181  thus contains option  234  indicating whether not the newly provisioned virtual disk desires de-duplication or not. 
     In addition, the identities of the metadata nodes which store this metadata for the virtual disk is also sent to the controller virtual machine for placing into cache  181 . Thus, the controller virtual machine is aware of on which compute nodes of the storage platform the metadata for a particular virtual disk is stored as shown in  FIG. 13 . 
     Virtual Disks, De-Duplication Table and System Disk 
       FIG. 5  is a symbolic representation of a virtual disk  330  showing how data within the virtual disk is stored within the storage platform and will be referred to in the description of  FIG. 9 . As shown, the virtual disk has been provisioned as a disk holding up to 50 GB, and the disk has been logically divided into segments or portions of 16 GB each. Each of these portions is termed a “container,” and may range in size from about 4 GB up to about 32 GB, although a size of 16 GB works well. As shown, the first portion  332  is referred to as container one, or C 1 , while the latter portions  334 - 338  are referred to respectively as containers C 2 , C 3  and C 4 . Note that the final container may have a size less than the sizes of the other containers. Containers have a particular naming convention. In one implementation, the convention is that the first container of a virtual disk will have a name that is the concatenation of: virtual disk name, “$” and “1.” The second container of that virtual disk will have a nearly identical name except that the final symbol will be a “2.” In this fashion, by knowing the container name, one knows with which virtual disk the container is associated. In addition, by knowing the virtual disk name, and an offset and size, one will be able to determine the names of the containers associated with a particular write request. In this fashion, each container name is unique within the entire storage platform. 
     Similar to a traditional hard disk, as data is written to the virtual disk the virtual disk will fill up symbolically from left to right, that is, first filling container C 1  and then container C 2 , etc. Each container of data will be stored upon a particular node or nodes within the storage platform that are chosen during the write process. If no replication is chosen, then data within a particular container will be stored on one node, while if replication is two, for example, then data within that container will be stored upon two nodes. In the example of  FIG. 5 , the replication factor is three, thus, data stored within container  332  will be stored upon the three nodes A, B and F. Any data stored within the second container  334  will be stored upon the three nodes B, D and E. Note that the set of nodes  342  and  344  might be the same nodes, may be completely different nodes, or may have nodes in common. 
     It is possible that when a particular file (for example) is written from a virtual machine to a virtual disk on the storage platform that the file may be written to two different containers, meaning that the file could potentially be stored upon six different computer nodes if the replication factor is three. For example, if virtual disk  330  already contains 12 GB of data and an 8 GB file is to be written, then this file will span containers C 1  and C 2  because the division between the containers is at 16 GB. Thus, the first 4 GB of file will be written into the nodes designated for container C 1 , while the second 4 GB of the file will be written into the nodes designated for container C 2 . Note that this storage technique using containers is an implementation of the storage platform and is totally transparent to the virtual machines that are storing data. In this fashion, use of the containers spreads the storage of a virtual disk throughout many different computer nodes within the storage platform. 
     In order to assist with the de-duplication of data on virtual disks within the storage platform, a de-duplication table is used as well as a system de-duplication disk. These structures are only used with virtual disks for which de-duplication has been turned “on.” 
       FIG. 6  illustrates a de-duplication table  310  that is maintained by the metadata module of each storage node. This table may be stored in the memory cache associated with each metadata module and may also be stored in persistent storage on the storage node. The same table may be stored and maintained by each metadata module within the platform. In a preferred embodiment, there will be three copies of the de-duplication table. Three nodes are chosen from the nodes in platform  20 , and that is where copies of de-duplication table  310  will reside. 
     Field  320  is the MD5 of a block of data that has been written to the storage platform for a virtual disk for which de-duplication has been turned on. Field  324  is the corresponding disk offset in the system de-duplication disk (explained below) for each MD5. Although the hash function MD5 is used, any other suitable hash function may be used to produce a unique number for a block of data. Accordingly, pairs of data in columns  320  and  324  form “key-value” pairs that indicate, for each MD5, where within the system de-duplication disk that particular block of data may be found. In one embodiment, table  310  is divided into 64 “buckets,” each bucket holding a range of pairs that are known by the CVM. The CVM can thus specify a certain bucket in the table to search in, thus helping with locking and unlocking when other CVMs access the same table. 
       FIG. 7  is an example of a system de-duplication disk  360  found within storage platform  20 . System disk  360  is a virtual disk similar to other virtual disks within the platform, except that it is hidden from clients and only accessible by the storage platform system software, typically the metadata and data modules. As will be explained in greater detail below, all data for any virtual disk that has de-duplication enabled will be written sequentially to system disk  360 . As shown, data may be written in blocks (typically, of size 4 k) at offsets shown at 0, 362, 364, etc. For example, if a first virtual disk writes a file of size 64 k that contains data unique to the storage platform, then all this data will be written sequentially onto system disk  360  in blocks of 4 k. De-duplication table  310  will then be updated to include the 16 MD5&#39;s for these blocks and their corresponding offsets within the system disk. If a second virtual disk then writes another file of size 64 k that contains the exact same data as in the first file, no data will be written to the storage platform because the data already exists. Rather, the metadata for this second file will be updated to indicate that the blocks of data for this second file can be found in the system disk and at the offsets previously stored in table  310  for the first file. 
     De-Duplication Write 
       FIG. 8  is a flow diagram describing one embodiment by which a virtual machine writes data to the storage platform that will not be duplicated. In this embodiment, an application on a virtual machine is writing to a virtual disk within the platform that has de-duplication enabled. The CVM is aware of which virtual disks are enabled for de-duplication and which are not because it has stored virtual disk information  260  into its memory cache  181 . For writes to a virtual disk for which de-duplication is not enabled, only steps  404 ,  416  and  420  need be performed. Note, though, that for a virtual disk in which de-duplication is not enabled, step  416  will write the data to the named virtual disk rather than to system disk  360 . 
     In step  404  the virtual machine (on behalf of its software application) that desires to write data into the storage platform sends a write request including the data to be written to a particular virtual disk. As mentioned, a write request may originate with any of the applications on one of computer servers  50 - 52  and may use any of a variety of storage protocols. The write request typically takes the form: write (offset, size, virtual disk name). The parameter “virtual disk name” is the name of the virtual disk originally selected as option  224 . The parameter “offset” is an offset within the virtual disk (i.e., a value from 0 up to the size of the virtual disk), and the parameter “size” is the length of the data to be written in bytes. As mentioned above, the CVM will trap or capture this write request sent by the application (in the block protocol or NFS protocol, for example). 
     Next, in step  408  the CVM calculates the MD5 of each block within the data to be written. Blocks may be of any size, although typically the size is 4 k bytes. After all of the message digests have been calculated (or perhaps after each one is calculated), in step  412  the CVM contacts one of the metadata modules executing on any one of the storage nodes within the storage platform in order to determine if each MD5 exists within de-duplication table  310 . If the MD5 exists, this indicates that that exact block of data has already been written somewhere in the storage platform for any virtual disk and that it will not be necessary to write that block of data again. If the MD5 does not exist, this indicates that the block of data does not exist within the storage platform yet and that the data block should be written. It is possible that within the data requested to be written, that some blocks already exist within the platform and that some do not. It is also possible that the MD5s for certain blocks will be the same (e.g., if all of these blocks are entirely filled with zeros), in this situation, only one request of the metadata module need be performed for this single MD5. For each query of table  310  with an MD5, the result returned from the metadata module is whether or not the MD5 exists, and if it exists, the system disk offset  324  is also returned. 
     In an alternative embodiment, the de-duplication table  310  is also stored within a cache of the CVM, for example, in persistent storage  195 . Thus, before step  412 , the CVM first checks this cache in order to determine if each MD5 exists within the table. If an MD5 is found, then it is not necessary to contact a metadata node for that particular MD5. If an MD5 is not found, then it may still be necessary to contact a metadata node as this cache may not include the entire table  310  because of a size limitation on the cache. When the entire table  310  is also stored within the cache, then it will not be necessary to contact a metadata node for any MD5. 
     For those blocks of data that do not already exist within the platform, step  416  will write those unique blocks to system disk  360  (rather than to the virtual disk named in the request of step  404 ) as is described below with reference to  FIG. 9  as modified as follows. As mentioned earlier, in the system disk, all writes are performed sequentially. Thus, wherever the last write to the system disk left off, that is where the new write begins. The CVM knows at which offset to start writing the blocks in the system disk because for a given virtual disk to which a block was previously written to the virtual disk (e.g., the answer to step  412  was “N”), the system first determines the last system disk offset (and length) where previous blocks were written. Then, the system determines the next set of available system disk blocks and that is where we end up writing new unique data. The last system disk offset used may be stored by the CVM in a variety of locations, including in conjunction with table  310 , within each CVM, within the virtual machine that sent the last write request, or in the metadata module of the storage nodes. 
     In one embodiment, the CVM periodically requests the next N offsets from the system disk that are available in which to write data blocks. For example, the CVM requests 1,000 offsets when needed. Thus, the CVM is aware of the available offsets and which offset was used last. This information regarding the next available N offsets and the last offset written to in the system disk may also be stored in the persistent storage in the appropriate storage nodes using the metadata module  130 . Thus, in a situation where multiple CVMs need to write blocks to the system disk, each can access this persistent storage to determine which offsets are available and to which offset a block was written last. Once a CVM writes a block to the system disk at a particular offset, it updates this persistent storage so that the offset information is always up to date for the next CVM requesting a write. 
     Next, for those unique blocks written in step  416  and for those blocks of data for which an MD5 exists in the duplication table  310 , their metadata will be written in step  420 . In step  420  the CVM calculates the block identifiers (i.e., blocks 1, 2, 3) within the system virtual disk where the data has been stored and then saves this information into the metadata nodes associated with the virtual disk. As shown in  FIG. 13 , the CVM knows which are the metadata nodes. As is known in the art, disks are typically divided up into blocks (usually blocks of 4 k) and data is written to, and read from, disks using blocks. Because the CVM is aware of the offset for the write request, the CVM then knows the block identifier for the first block to be written for the current write request. And, because the size of the write request is also known, the CVM is then able to easily calculate onto which data nodes blocks of data were written, and the corresponding block identifiers for those blocks of data. In the current example, the CVM calculates the block identifiers for those blocks of data in the current write request which were written to nodes B, D and E (for example). Even if a write request spans two different containers, by simple calculation using the container size, offset, and size of the write request, the CVM will be able to determine which block identifiers were written to the first container and which block identifiers were written to the second container. For writes of blocks to the system disk which are not contiguous, the CVM is able to determine the block identifiers that were written. 
     The CVM then stores these block identifiers into the three metadata nodes holding the metadata for the named virtual disk as shown in  FIG. 11  as described below. Thus, these three metadata nodes are always aware which block identifiers have been written for this particular virtual disk, and where. Of course, other virtual disks that have been created within the storage platform may use different sets of three metadata nodes (or perhaps fewer or more metadata nodes) in order to store their metadata. In this step the locations of the containers (nodes and storage pools) are also written into the metadata modules associated with that virtual disk. And, the CVM sends the current version of the virtual disk with the block identifiers so that the current version may be stored along with the block identifiers in the metadata nodes as shown in  FIG. 11 . For non de-duplication enabled disks, the CVM also generates a timestamp and includes this timestamp along with the block identifiers so that each block includes a timestamp in metadata. For de-duplication enabled disks, a system disk offset is stored for each block. 
     Preferably, write operations do not overwrite older versions of data. In this fashion, earlier versions of data in a virtual disk are always available to be read. Thus, snapshot and revert operations can be performed very quickly because data does not need to be copied into special storage locations or retrieved from special storage locations. All versions of data are always available within the storage platform. As shown in  FIG. 11 , blocks of data are stored as “chunks,” each chunk including in its name the version number which identifies the version of the data stored. 
     Finally, in step  424 , the duplication table  310  is updated on all of the metadata modules where the table is stored. Because the unique blocks were written to the system disk  360  in step  416 , the MD5 for each of these blocks will be added to table  310  along with the corresponding system disk offset  324  for that particular block. Thus, in this fashion, the MD5s for all newly written blocks will be available in the de-duplication table in order to prevent those blocks from being written again into the storage platform. In the alternative embodiment in which a cache of the CVM stores a portion of table  310 , in addition to step  424 , table  310  will also be updated in the cache to include the MD5s for all newly written blocks along with each corresponding system disk offset. 
       FIG. 9  describes one embodiment by which a CVM writes unique blocks to a virtual disk (a user virtual disk or the system virtual disk) in the storage platform. In step  508  the controller virtual machine determines which containers to use for this request based upon the offset and size parameters. For example, because the CVM is aware of the size of each container (typically 16 GB), knowing the offset and the size of the write request, the CVM is able to determine which container shall be used and whether more than one container is needed. 
     Step  404  provides the offset for a user virtual disk, but, in cases of a de-duplication-enabled virtual disk, and where the answer to step  412  is “No,” an offset for the system disk may be determined as follows. For writing to the system virtual disk, we write sequentially. First we determine the last system disk offset (and length) that we wrote any block with. Then, we determines the next set of available system disk blocks and that is where we end up writing new unique data. 
     Each container is provided with a unique identifier within the platform, and containers used to write to other virtual disks will also each have an identifier unique within the platform. Assuming that only one container is needed (for example, C 2 , as shown in  FIG. 5 ), the CVM then proceeds to determine on which computer nodes of the storage platform the data should be stored. 
     In step  512  the CVM queries a metadata node to determine on which computer nodes the container should be stored. Because the particular metadata nodes on which the metadata for the virtual disk is stored had been previously cached by the CVM (see, e.g.,  FIG. 13 ), the CVM can easily select one of these metadata nodes to query. The CVM sends a query with the container to be used (e.g., C 2 ) and requests the return of a list of the actual data nodes on which to write that container. If that container had previously been written to that virtual disk then the metadata node knows which data nodes to return because it had previously stored that information. If this is a first write request for a particular container, then the metadata node determines which and how many computer nodes to assign to that container based upon how many replicas are needed. Once the metadata node retrieves or determines the data nodes to use for the write request, it then returns a list to the CVM (e.g., it returns a list of these nodes  344 : B, D and E). The CVM is now ready to perform the write request. 
     In step  516  the CVM then sends the write request (the data itself to be written and the offset in the system disk for each block if the virtual disk is de-duplication enabled) to one of the data nodes returned in the previous step (e.g., data node E). The write request also includes an indication of the other two data nodes (B, D) to which the data should be written. The data node that receives the request then writes the data to its disk drives and then forwards the data to the other two nodes. Once each of these nodes writes the data to its disk drives, each of these nodes returns an acknowledgment back to the first data node that had originally received the request from the CVM. The CVM is able to specify the system disk offset because it fetches the next available offset for the system disk from one of the storage nodes, and then issues a system disk write at that offset. 
     In one situation when a write is performed to the system disk on behalf of a virtual disk with de-duplication enabled, it is possible that some of the blocks will be exactly the same, e.g., they are all entirely zeros. In this situation, only a single write of one of these blocks to a storage node need be performed; the metadata of all of these duplicate blocks will indicate where the one block was written to the storage node. The determination that all of these blocks are the same was made above when the MD5s were calculated. 
     The CVM also sends with the write request the relevant policies for the particular virtual disk. For example, the CVM indicates how many replicas are needed, the type of storage disk required, whether the data should be compressed, what the replication policy is, etc. The data node that receives the request will then handle implementations of all of these policies. The CVM does not need to concern itself with how these policies are implemented; it simply sends the write request with the relevant information. 
     Also in step  516 , because the CVM has a cache  181  that contains the current version and version tree for each virtual disk that is attached to it (i.e., for each virtual disk used by the virtual machines on the same computer as the CVM), the CVM is also able to send the current version of the virtual disk with the write request so that as blocks of the virtual disk are written onto their data nodes the current version may be stored along with each block. A timestamp is also sent with the write request. 
     In addition to writing the data over an Internet connection to data nodes within a remote storage platform  20 , the CVM also writes the same data blocks into solid-state storage  195  (a block cache) so that the data blocks may be read from this cache much more quickly if needed. The data blocks are identified within this cache preferably using the block identifiers shown in  870 . 
     In step  520  this first data node (e.g., E) acknowledges that the write has occurred to the CVM and returns the names of the data nodes (e.g., B, D and E) where the data was written, the system disk offset for each block (for de-duplication disks) and a timestamp for each block (for non de-duplication disks). 
     Storage of Metadata 
       FIGS. 10 and 11  illustrate metadata storage within platform  20 . The metadata used within storage system  10  encompasses many types and includes: virtual disk information; container location information (including on which storage pools the containers are located and on which nodes); storage pool information (including which nodes have which pools and which disks); and, mutation information after write requests (where were blocks written, success and failure, etc.). As mentioned earlier, while the data associated with a particular write request may end up on three different data nodes, the metadata information associated with virtual disks, write requests, etc. will be stored using the metadata modules  130  on the computer nodes, and these nodes may be different from the nodes used for data storage. 
       FIG. 10  illustrates three types of information stored by a metadata module. Each of these types of information may use any suitable data structure and each type of information is independent of the others and may use a different data structure. Shown, for example, is persistent storage on node A that has been stored by its metadata module. Shown at  860  is the storage pool information for that node, showing which storage pools are present on that node and which disks are associated with each storage pool. Shown at  861  is container location information. Using the naming convention for containers discussed above, this shows that container “Vi$2” (associated with virtual disk “Vi”) is located on node A and that it is part of storage pool SP 3 . The second listed container Vi&amp;7 is shown as being part of the storage pool SP 2 . Of course, many other containers that are stored on node A may also be listed in this data structure. Shown at  862  are all of the policies and other information for the virtual disk named “Avinash.” Of course, any number of other virtual disks may also have their information or metadata  864  stored on this particular node. 
       FIG. 11  illustrates another type of information stored by a metadata module. Shown, for example, is persistent storage on node D that has been stored by its metadata module. Shown symbolically (not to scale) at  870  is a virtual disk showing how its stored information is represented within metadata storage. In this example, assume that the virtual disk has a size of 1 TB, that each chunk portion has a size of 256 kB, assume that each block has a size of 4 kB, and that  66  blocks have been written into this virtual disk. Chunks  871  and  872  illustrate that metadata is stored on a per chunk basis. 
     For virtual disk  870 , metadata storage region  880  is shown; write information is stored in columns  882 ,  884 , etc., each column corresponding to a particular chunk of the virtual disk. (Other virtual disks will also have similar storage regions with their own metadata.) Column  882  represents the first chunk and also includes the version number. Column  884  represents the second chunk. In this embodiment, there will be a new column if the version is incremented and one writes again into the first chunk. In this fashion, older versions of data are never overwritten or lost, they are all saved within the storage platform for later reference if necessary. 
     Within each chunk column  882 , are individual block columns  890 ,  892 , etc., representing the individual blocks of that chunk that have been written to the virtual disk. For example, column  890  includes the block identifier “1,” the computer nodes to which that block was written, whether not the write was a success, and a timestamp. Column  892  includes similar information for the second block. Within column  882  there will be 64 individual block columns due to the size of the blocks and the size of the chunks. Column  884  will also include the same number of block columns, for example, block column  896  identifies the block identifier “66,” and the information earlier described. In this fashion, the metadata for particular virtual disk  880  is stored upon one of the computer nodes using its metadata module, and includes an identification of where each of its blocks were stored, a version, a timestamp, an identifier for each block, etc. 
     De-Duplication Read 
       FIG. 12  is a flow diagram describing one embodiment by which a virtual machine reads data from the storage platform. In this embodiment, an application on a virtual machine is reading from a virtual disk within the platform that may or may not have de-duplication enabled. 
     In step  664  the virtual machine that desires to read data from the storage platform sends a read request from a particular application to the desired virtual disk. As explained above, the controller virtual machine will then trap or capture the request (depending upon whether it is a block request or an NFS request) and then place a request into its own protocol before sending the request to the storage platform. 
     As mentioned, a read request may originate with any of the virtual machines on computers  50 - 52  (for example) and may use any of a variety of storage protocols. The read request typically takes the form: read (offset, size, virtual disk name). The parameter “virtual disk name” is the name of a virtual disk on the storage platform. The parameter “offset” is an offset within the virtual disk (i.e., a value from 0 up to the size of the virtual disk), and the parameter “size” is the length of the data to be read in bytes. The CVM divides up the request into blocks; e.g., a request of size 64 k is divided up into sixteen requests of 4 k each, each request having a corresponding offset within the named virtual disk. 
     In addition to block cache  195 , each CVM may also utilize a separate “de-duplication” cache that may be used to cache blocks for a de-duplication-enabled disk. A table such as that shown in  FIG. 6  may be used to keep track of an identifier for each such block and its corresponding system disk offset. Thus, in one embodiment, the CVM first checks this de-duplication cache to determine whether any of the blocks to be read have already been written and are already present within this cache. If so, these blocks are retrieved from this block cache instead of having to establish a remote connection with storage platform  20  and retrieve those blocks remotely (which would take a greater deal of time). All blocks not found in this de-duplication cache are read from the storage nodes as described below. 
     Next, in step  668  the controller virtual machine determines which container or containers to read from for each block of this request based upon the offsets determined as described above, and then determines the relevant metadata nodes for this virtual disk, as shown in  FIG. 13 . The CVM also determines the block identifiers that it needs to read using the offset and the size from the request. 
     In step  672  the CVM queries a metadata node to determine on which computer nodes the data blocks identified by the block identifiers are written. Because the particular metadata nodes on which the metadata for the virtual disk is stored had been previously cached by the CVM, the CVM can easily select one of these metadata nodes to query. The CVM sends a query with a container identifier (e.g., C 2 ) and requests the return of a list of the actual computer nodes on which the data had been written. The metadata node knows which data nodes to return because it had previously stored that information in step  420 ; see, for example,  FIG. 11 . For data block  1 , for example, metadata region  880  indicates that the block is stored on data nodes A, B and C, and that a system disk offset for that data block is found in field  888 . Virtual disks with de-duplication enabled will have a system disk offset stored in field  888 ; those with de-duplication not enabled will have a timestamp stored in column  890 , rather than a system disk offset. Once the metadata node retrieves the data nodes where the data blocks are stored, along with a system disk offset for each block (or a timestamp for non-enabled virtual disks) it then returns this information to the CVM. The CVM is now ready to perform the read request. 
     In step  676  the CVM then sends the read request to each of the data nodes returned in the previous step. The read request includes a list of block identifiers to be read, the system disk offset for each block (for a de-duplication-enabled virtual disk), and a timestamp (for a non de-duplication-enabled virtual disk). 
     In one situation involving a read from the system disk on behalf of a virtual disk with de-duplication enabled, it is possible that some of the blocks to be read may be the same (e.g., they are all entirely zeros). In this situation, all of the system disk offsets for these blocks will all point to the same block on disk. It is only necessary to perform a single read for all of these system disk offsets in order to obtain the single block. 
     In step  680  each data node returns the data identified by the block identifiers to the CVM. In step  684  the CVM then returns this data to the requesting virtual machine using the appropriate protocol, again masquerading either as a block device or as an NFS device depending upon the protocol used by the particular application. 
     Alternatively, the read request is sent to only one of the data nodes (e.g., B) and also includes an indication of the other two data nodes (D, E) from which the data should be read. The first data node then passes the read request to each of the other data nodes. Each of these data nodes that receives the request then reads the data from its disk drives. Once each of these nodes reads the data from its disk drives, each of these nodes returns the data back to the first data node that had originally received the request from the CVM. This first data node then returns the data to the CVM. 
       FIG. 13  shows information that may be stored within memory cache  181  of a controller virtual machine (CVM)  180  on one of the computer servers. Preferably, all information concerning a particular virtual disk attached to a CVM will be organized into a virtual disk object and then stored into the memory cache. A hash table is used to store these virtual disk objects and the key to find each object is the name of the virtual disk. Stored within this cache is the generation number  846 , virtual disk information  260  and metadata nodes  898  indicating on which nodes the metadata for this virtual disk is stored. 
     Computer System Embodiment 
       FIGS. 14 and 15  illustrate a computer system  900  suitable for implementing embodiments of the present invention.  FIG. 14  shows one possible physical form of the computer system. Of course, the computer system may have many physical forms including an integrated circuit, a printed circuit board, a small handheld device (such as a mobile telephone or PDA), a personal computer or a super computer. Computer system  900  includes a monitor  902 , a display  904 , a housing  906 , a disk drive  908 , a keyboard  910  and a mouse  912 . Disk  914  is a computer-readable medium used to transfer data to and from computer system  900 . 
       FIG. 15  is an example of a block diagram for computer system  900 . Attached to system bus  920  are a wide variety of subsystems. Processor(s)  922  (also referred to as central processing units, or CPUs) are coupled to storage devices including memory  924 . Memory  924  includes random access memory (RAM) and read-only memory (ROM). As is well known in the art, ROM acts to transfer data and instructions uni-directionally to the CPU and RAM is used typically to transfer data and instructions in a bi-directional manner. Both of these types of memories may include any suitable of the computer-readable media described below. A fixed disk  926  is also coupled bi-directionally to CPU  922 ; it provides additional data storage capacity and may also include any of the computer-readable media described below. Fixed disk  926  may be used to store programs, data and the like and is typically a secondary mass storage medium (such as a hard disk, a solid-state drive, a hybrid drive, flash memory, etc.) that can be slower than primary storage but persists data. It will be appreciated that the information retained within fixed disk  926 , may, in appropriate cases, be incorporated in standard fashion as virtual memory in memory  924 . Removable disk  914  may take the form of any of the computer-readable media described below. 
     CPU  922  is also coupled to a variety of input/output devices such as display  904 , keyboard  910 , mouse  912  and speakers  930 . In general, an input/output device may be any of: video displays, track balls, mice, keyboards, microphones, touch-sensitive displays, transducer card readers, magnetic or paper tape readers, tablets, styluses, voice or handwriting recognizers, biometrics readers, or other computers. CPU  922  optionally may be coupled to another computer or telecommunications network using network interface  940 . With such a network interface, it is contemplated that the CPU might receive information from the network, or might output information to the network in the course of performing the above-described method steps. Furthermore, method embodiments of the present invention may execute solely upon CPU  922  or may execute over a network such as the Internet in conjunction with a remote CPU that shares a portion of the processing. 
     In addition, embodiments of the present invention further relate to computer storage products with a computer-readable medium that have computer code thereon for performing various computer-implemented operations. The media and computer code may be those specially designed and constructed for the purposes of the present invention, or they may be of the kind well known and available to those having skill in the computer software arts. Examples of computer-readable media include, but are not limited to: magnetic media such as hard disks, floppy disks, and magnetic tape; optical media such as CD-ROMs and holographic devices; magneto-optical media such as floptical disks; and hardware devices that are specially configured to store and execute program code, such as application-specific integrated circuits (ASICs), programmable logic devices (PLDs) and ROM and RAM devices. Examples of computer code include machine code, such as produced by a compiler, and files containing higher-level code that are executed by a computer using an interpreter. 
     Although the foregoing invention has been described in some detail for purposes of clarity of understanding, it will be apparent that certain changes and modifications may be practiced within the scope of the appended claims. Therefore, the described embodiments should be taken as illustrative and not restrictive, and the invention should not be limited to the details given herein but should be defined by the following claims and their full scope of equivalents.