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Snapshot constraint

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5.4.4 Snapshot constraint D-LINK 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. Figure 5.4.4.1 93

Section	Page
Chapter 1 Overview	6
1.1 Features	6
1.1.1 Highlights	7
1.2 RAID concepts	8
1.2.1 Terminology	8
1.2.2 RAID levels	10
1.2.3 Volume relationship	11
1.3 iSCSI concepts	12
1.4 IP SAN storage specifications	13
1.4.1 Technical specifications	13
1.4.2 FCC and CE statements	16
Chapter 2 Installation	19
2.1 Package contents	19
2.2 Before installation	19
2.3 Enclosure	19
2.3.1 Front view	19
2.3.2 Front LED lights	20
2.3.3 Install drives	21
2.3.4 Rear view	22
2.4 Install battery backup module	25
2.5 Deployment	26
Chapter 3 Quick setup	29
3.1 Management interfaces	29
3.1.1 Serial console	29
3.1.2 Remote control	29
3.1.3 Web UI	30
3.2 How to use the system quickly	32
3.2.1 Quick installation	32
3.2.2 Volume creation wizard	35
Chapter 4 Configuration	38
4.1 Web UI management interface hierarchy	38
4.2 System configuration	39
4.2.1 System setting	39
4.2.2 Network setting	40
4.2.3 Login setting	41
4.2.4 Mail setting	42
4.2.5 Notification setting	43
4.3 iSCSI configuration	45
4.3.1 NIC	45
4.3.2 Entity property	48
4.3.3 Node	48
4.3.4 Session	51
4.3.5 CHAP account	52
4.4 Volume configuration	53
4.4.1 Physical disk	54
4.4.2 RAID group	57
4.4.3 Virtual disk	60
4.4.4 Snapshot	65
4.4.5 Logical unit	68
4.4.6 Example	69
4.5 Enclosure management	74
4.5.1 Hardware monitor	75
4.5.2 UPS	76
4.5.3 SES	78
4.5.4 Hard drive S.M.A.R.T.	78
4.6 System maintenance	79
4.6.1 System information	79
4.6.2 Event log	80
4.6.3 Upgrade	82
4.6.4 Firmware synchronization	82
4.6.5 Reset to factory default	83
4.6.6 Import and export	83
4.6.7 Reboot and shutdown	84
4.7 Home/Logout/Mute	84
4.7.1 Home	84
4.7.2 Logout	84
4.7.3 Mute	85
Chapter 5 Advanced operations	86
5.1 Volume rebuild	86
5.2 RG migration	88
5.3 VD extension	89
5.4 Snapshot	89
5.4.1 Create snapshot volume	90
5.4.2 Auto snapshot	91
5.4.3 Rollback	92
5.4.4 Snapshot constraint	93
5.5 Disk roaming	95
5.6 VD clone	95
5.7 SAS JBOD expansion	103
5.7.1 Connecting JBOD	103
5.7.2 Upgrade firmware of JBOD	105
5.8 MPIO and MC/S	106
5.9 Trunking and LACP	108
5.10 Dual controllers (only for DSN-6410 with DSN-640)	109
5.10.1 Perform I/O	109
5.10.2 Ownership	110
5.10.3 Controller status	111
5.11 Replication	112
5.12 VLAN	122
Chapter 6 Troubleshooting	125
6.1 System buzzer	125
6.2 Event notifications	125
Appendix	133
A. Certification list	133
B. Microsoft iSCSI initiator	136

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5.4.4

Snapshot constraint

D-LINK

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.

Figure 5.4.4.1