Qsan AegisSAN Q500 Series, Руководство пользователя

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Qsan Document – User Manual
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 . Qsan storage system can support Microsoft VSS.

What if the snapshot space is over?
Before using snapshot, a snapshot space is needed from RAID group 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 virtual disk 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 virtual disk?
There are up to 64 snapshots can be created per virtual disk. What if the 65th snapshot has
been taken? There are two different situations:
1. If the snapshot is configured as schedule snapshot, the latest one (the 65th snapshot)
will replace the oldest one (the first snapshot) and so on.
2. If the snapshot is taken manually, when taking the 65th snapshot will fail and a
warning message will be showed on Web UI.

Rollback and delete snapshot
When a snapshot has been rollbacked, the related snapshots which are earlier than it will
also be removed. But the rest snapshots will be kept after rollback. If a snapshot has been
deleted, the other snapshots which are earlier than it will also be deleted. The space
occupied by these snapshots will be released after deleting.