NOTE: Snapshot capacity must be reserved on (from the original RAID Group) when creating a Virtual Disk. It is not possible to assign Snapshot space/capacity to a Virtual Disk after the Virtual Disk has been created.
Snapshot copies may consume disk space greater than the reserve amount. It is important to reserve enough disk space for future Snapshots or to modify existing copies.
What happens when snapshot copies exceed the reserve capacity?
There are two different situations:
1. Single Snapshot The Snapshot write process will fail due to lack of disk space.
2. Multiple Snapshots or Copies
The system will begin to overwrite existing Snapshots, starting with the oldest Snapshot copy (Snapshot No.1), until enough capacity has been cleared to allow a new Snapshot to be written.
When user modifies the data from host, the data will pass through file system and memory of the host (write caching). Then the host will flush the data from memory to physical disks, no matter the disk is local disk (SATA), DAS (SCSI or SAS), or SAN (Fibre or iSCSI). From the viewpoint of storage device, it cannot control the behavior of host side. This case maybe happens. If user takes a snapshot, some data is still in memory and not flush to disk. Then the snapshot may have an incomplete image of original data. The problem does not belong to the storage device. To avoid this data inconsistent issue between snapshot and original data, user has to make the operating system flush the data from memory of host (write caching) into disk before taking snapshot.
ULTRASTOR supports up to 32 snapshots for each Virtual Disk.
1. Automatic Snapshots: Once the Snapshot limit has been exceeded, the system will be begin to cycle through existing Snapshots (Snapshot No.33 will replace the first snapshot Snapshot No.1, and so on)
2. Manual Snapshots: Once the Snapshot limit has beed exceeded, the following Snapshot (Snapshot No. 33) will fail and a generate a warning message/notification.
On Linux and UNIX platforms, the sync command can be used to make the operating system flush data from write caching into disk. For Windows platform, Microsoft also provides a tool – sync, which can do exactly the same thing as the sync command in Linux/UNIX. This feature will tell the OS to flush the data on demand.
For more detail about sync tool, please visit: http://technet.microsoft.com/enus/sysinternals/bb897438.aspx
Besides the sync tool, Microsoft develops VSS (volume shadow copy service) to prevent this issue. VSS is a mechanism for creating consistent point-in-time copies of data known as shadow copies. It is a coordinator between backup software, application (SQL or Exchange...) and storages to make sure the snapshot without the problem of data-inconsistent.
For more detail about the VSS, please visit: http://technet.microsoft.com/en-us/library/cc785914.aspx