IBM XIV Storage System Copy Services and Migration

►Redirect-on-write (differential copy)A redirect-on-write shadow copy volume is a differential copy (rather than a full copy) ofthe original data as it resides on a volume. This method is similar to copy-on-write, withoutthe double-write penalty, and it offers storage space and performance efficient snapshots.New writes to the original volume are redirected to another location set aside for snapshot.The advantage of redirecting the write is that only one write takes place, whereas withcopy-on-write, two writes occur (one to copy original data onto the storage space, theother to copy changed data). The XIV storage system supports redirect-on-write.2.3.2 Microsoft Volume Shadow Copy Service functionMicrosoft VSS accomplishes the fast backup process when a backup application (therequestor, which is Tivoli Storage FlashCopy Manager in our case) initiates a shadow copybackup. The VSS service coordinates with the VSS-aware writers to briefly hold writes ondatabases, applications, or both. VSS flushes the file system buffers and asks a provider(such as the XIV provider) to initiate a snapshot of the data. When the snapshot is logicallycompleted, VSS allows writes to resume and notifies the requestor that the backup hascompleted successfully. The (backup) volumes are mounted, but hidden and read-only, readyto be used when a rapid restore is requested. Alternatively, the volumes can be mounted to adifferent host and used for application testing or backup to tape.The Microsoft VSS FlashCopy process is:1. The requestor notifies VSS to prepare for a shadow copy creation.2. VSS notifies the application-specific writer to prepare its data for making a shadow copy.3. The writer prepares the data for that application by completing all open transactions,flushing cache and buffers, and writing in-memory data to disk.4. When the application data is ready for shadow copy, the writer notifies VSS, which in turnrelays the message to the requestor to initiate the commit copy phase.5. VSS temporarily quiesces application I/O write requests for a few seconds and the VSShardware provider performs the snapshot on the storage system.6. Once the storage snapshot has completed, VSS releases the quiesce, and database orapplication writes resume.7. VSS queries the writers to confirm that write I/Os were successfully held during theVolume Shadow Copy.2.4 XIV VSS providerA VSS hardware provider, such as the XIV VSS Provider, is used by third-party software toact as an interface between the hardware (storage system) and the operating system. Thethird-party application (which can be IBM Tivoli Storage FlashCopy Manager) uses XIV VSSProvider to instruct the XIV storage system to perform a snapshot of a volume attached to thehost system.Chapter 2. Tivoli Storage FlashCopy Manager and Volume Shadow Copy Services 47