Overview

Snapshot is a technology that backs up data by capturing the storage's file system at a specific point in time like taking a picture. If the latest data is damaged due to an accident during replication operation or data integrity is compromised due to exposure to security issues such as malware infection, it is difficult to respond only with the replication function. If you have backed up your data with snapshots in advance, you can avoid the worst by recovering to a specific point in time. In other words, the FSR snapshot function serves as an auxiliary function to replication.

Snapshots are stored as images within each node's disk volume and are individually controlled and processed within the node. This means that snapshots between cluster nodes are not interoperable. In other words, if you need to restore snapshots after operating snapshots for each node, you can restore individual nodes.

Configurations

Environment

The first thing to consider before operating the snapshot is to designate the volume where the snapshot will be stored. Snapshots can be kept within the clone volume or stored externally on another disk volume. This should be determined by looking at the used space and free space on the volume. If there is not much free space, it is recommended to specify an external volume to save the snapshot. 

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Snapshots are based on Copy On Write (COW) technology. It is a way to save the original when changes occur to the data. Therefore, in the early days, not long after the snapshot was created, the capacity of the snapshot is small because there are not many changes. However, as time passes, the number of data changes gradually increases, and assuming that all areas of data are changed, snapshot space is required to store the entire original data might be required. In conclusion, we can say that the maximum capacity required for a snapshot is equal to the capacity of a full backup of your data. 

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Snapshot type

To operate a snapshot, you need to specify the snapshot type. Snapshots have copy, full, differential, and incremental options depending on the recording method, and are specified in the type item of the snapshot section of the configuration file.

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In FSR, the copy type is specified by default, so if there is no special request, snapshots are recorded and backed up with the copy type. However, since copy backups cannot serve as a basis for differential and incremental backups, differential and incremental backups may require that they be written as full backups first. This depends on the type of backup supported by the application.

Pre/Post Processing

FSR snapshots are oriented toward application consistency. In order for the application to acquire consistent snapshots, the following procedures must be performed.

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If the application cannot be controlled by the above procedure, at least the file system cache should be flushed and recorded as a snapshot with file system consistency. will be

Control

FSR's snapshot function is an additional function to perform backup of the replication target during replication operation. In other words, snapshots are managed and controlled in units of replication resources and operated as sub-concepts of replication resources. 

Each control command is as follows.

Check environments for support

First, check if your current environment supports the snapshot feature.

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As above in the status output command, if the resource's snapshot status ( snapshotitem ) unavailableis not an available environment.

create and delete

Snapshots can be created after configuring the clone resource, meta-create, and starting up. The created snapshot is managed by being included in the meta information of FSR, and is associated with the FSR resource until the meta information is deleted. Snapshot images are not deleted when the resource is deleted. Explicitly deleting the snapshot or clearing the meta disassociates the resource from the snapshot.

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check

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restore

restore

Snapshot recovery must be preceded by the following operations.

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Because restore commands are performed asynchronously, like snapshot creation, progress must be verified through status queries.

export

Executing the above command copies the contents of the snapshot to the specified path. If there are multiple replication destination paths, you can export by specifying multiple destination paths. If exporting all replication destination paths to one path, you only need to specify options --combineand create one destination path.

expose

You can directly access the file by connecting the created snapshot to the specified path. However, since a snapshot can have more than one image, you must enter the index (number) of the image. The image list can be checked through the snapshot lookup command.

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cleanup

Snapshots that are no longer valid due to the snapshot image being deleted can be deleted in bulk with the following command.

Troubleshootings

Windows

Backup type not supported error

If the backup type specified in the configuration file is not supported by the VSS writer installed in the operating system, an error may occur during the creation process. Here is the error that occurs when SqlServerWriter, the VSS writer for MS SQL, does not support incremental backups, but FSR specifies to do incremental backups.

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You need to change to one of the backup types printed in the error and create a snapshot.

Writer error

If the VSS writer encounters an error during snapshot creation, no details are communicated to the VSS requester (FSR). For example, when an error occurs in the VSS writer in MS SQL, the following error is output:

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