D-Link DSN-6210 User Manual - Page 113

Snapshot Constraint, CAUTION, Before executing rollback

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D-Link 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 disks and provides a quick and efficient backup method. When taking a snapshot, it does not copy any data the first time until a request of data modification comes in. The snapshot copies the original data to a snapshot space and then overwrites the original data with new changes. With this technique, a snapshot only copies the changed data instead of copying whole data, saving you a lot of disk space.  Create a data-consistent snapshot Before using snapshot, the user has to know why sometimes the data corrupts after rollback of snapshot. Please refer to the following diagram. When the user modifies the data from the host, the data will pass through the file system and the memory of the host (write caching). Then the host flushes the data from memory to physical disks, no matter if the disk is local disk, DAS, or SAN. From the viewpoint of the storage device, it cannot control the behavior on the host side. If a snapshot is taken, and some data is still in memory and not on the disk, then the snapshot may have an incomplete image of the original data. The problem does not belong to the storage device. To avoid this data inconsistency issue between snapshot and original data, the user has to make the operating system flush the data from the memory of the host (write caching) into the disk before taking a snapshot. Copyright@2014 D-Link System, Inc. All Rights Reserved. 113

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D-Link 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 disks and provides a quick and
efficient backup method. When taking a snapshot, it does not copy any data the first time until a
request of data modification comes in. The snapshot copies the original data to a snapshot space
and then overwrites the original data with new changes. With this technique, a snapshot only
copies the changed data instead of copying whole data, saving you a lot of disk space.
Create a data-consistent snapshot
Before using snapshot, the user has to know why sometimes the data corrupts after rollback of
snapshot. Please refer to the following diagram.
When the user modifies the data from the host, the data will pass through the file system and the
memory of the host (write caching). Then the host flushes the data from memory to physical disks,
no matter if the disk is local disk, DAS, or SAN. From the viewpoint of the storage device, it cannot
control the behavior on the host side. If a snapshot is taken, and some data is still in memory and
not on the disk, then the snapshot may have an incomplete image of the original data. The
problem does not belong to the storage device. To avoid this data inconsistency issue between
snapshot and original data, the user has to make the operating system flush the data from the
memory of the host (write caching) into the disk before taking a snapshot.
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Copyright@2014 D-Link System, Inc. All Rights Reserved.