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Copyright@2004~2014 Qsan Technology, Inc. All Rights Reserved.
Qsan Document – User Manual
CAUTION:
Before executing rollback, it is better that the disk is unmounted on the host
computer for flushing data from cache.
Snapshot Constraint
Snapshot function applies Copy-on-Write technique on virtual disk and provides a quick and
efficient backup methodology. When taking a snapshot, it does not copy any data at first time until
a request of data modification comes in. The snapshot copies the original data to snapshot space
and then overwrites the original data with new changes. With this technique, snapshot only copies
the changed data instead of copying whole data. It will save a lot of disk space.
Create a data-consistent snapshot
Before using snapshot, user has to know why sometimes the data corrupts after rollback of
snapshot. Please refer to the following diagram.
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 (IDE or 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 a snapshot is taken, 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.
