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On Linux and UNIX platform, a command named sync 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. It will tell the OS to flush the data on demand. For more detail about sync
tool, please refer to: http://technet.microsoft.com/en-us/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 refer to
http://technet.microsoft.com/en-us/library/
cc785914.aspx. DSN-6420/6410 can support
Microsoft VSS.
What if the snapshot space is over?
Before using snapshot, a snapshot space is needed from RG capacity. After a period of
working snapshot, what if the snapshot size over the snapshot space of user defined?
There are two different situations:
1. If there are two or more snapshots existed, the system will try to remove the oldest
snapshots (to release more space for the latest snapshot) until enough space is
released.
2. If there is only one snapshot existed, the snapshot will fail. Because the snapshot
space is run out.
For example, there are two or more snapshots existed on a VD and the latest snapshot
keeps growing. When it comes to the moment that the snapshot space is run out, the
system will try to remove the oldest snapshot to release more space for the latest
snapshot usage. As the latest snapshot is growing, the system keeps removing the old
snapshots. When it comes that the latest snapshot is the only one in system, there is no
more snapshot space which can be released for incoming changes, then snapshot will fail.
How many snapshots can be created on a VD
There are up to 32 snapshots can be created on a UDV/VD. What if the 33rd snapshot has
been taken? There are two different situations:
1. If the snapshot is configured as auto snapshot, the latest one (the 33rd snapshot)
will replace the oldest one (the first snapshot) and so on.
2. If the snapshot is taken manually, when taking the 33rd snapshot will fail and a
warning message will be showed on Web UI.
Rollback / Delete snapshot