User manual

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5.4.4 iSnap constraint

RAG362 snapshot function applies Copy-on-Write technique on UDV/VD 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 can not 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.

Caution

Before executing rollback, it is better to dismount file system for

flushing data from cache to disks in OS first. System sends pop-up

messa

e when user executes rollback function.

RAG362