Topics:
What is a Point-In-Time Copy?
Traditional Volume Snapshots
File System Snapshots
A point-in-time copy (PITC) enables you to capture an image of data at a selected instant for use in applications, such as backups, decision support, reporting, and development testing.
Physical vs. Logical PITCs
Physical PITCs –
The physical PITC is a physically distinct copy of the data usually produced by breaking off a mirror of the storage container.
Advantages:
Complete copy of the primary data
Fully synchronized
Disadvantages:
Requires the same amount of storage space as the original
Requires time for synchronization of data
Logical PITCs –
This PITC identifies and maintains modified blocks, and in addition, there is a reference to the original data. The logical PITC is dependent on the primary copy of the data.
Advantages:
Available for use instantaneously
Disadvantages:
Dependent on the original.
Performance Issues with Physical PITCs
The primary impact for physical PITCs is the initial synchronization. This is especially important when large amounts of data need to be copied.
After this full synchronization is complete, there is very little, if any performance impact on the original volume or the PITC because they are separate objects.
Performance Issues with Logical PITCs
The logical PITC is connected to the primary data. Therefore, the I/O of a logical PITC is subject to the rate of change of the original data. The overall impact of the PITC is dependent on the read-to-write ratio of an application and the mixing of the I/O operations.
Note: Both the primary data and the logical PITC become faster as more data is copied out from the primary, because the PITC slowly becomes a complete physical copy over time.
Life Cycle of Point-in-Time Copies
1) Make PITC (Assign Resources) – vxassist snapstart & vxassist snapshot
2) Use PITC (Testing, Backup, etc.)
3) Update PITC (update PITC with new data from the primary or repopulate the primary from the PITC) – vxassist [-o resyncfromreplica] snapback
4) Destroy PITC (Release Resources) – vxassist remove
Traditional Volume Snapshots
The traditional type of volume snapshot that was originally provided in VxVM is the third-mirror break-off type.
When you create a traditional volume snapshot, you create a temporary mirror of an existing volume. After the contents of the third mirror (or snapshot plex) are synchronized from the original plexes of the volume, the snapshot plex can be detached as a snapshot volume for use in backup or decision support applications.
Creating and Managing Traditional Volume Snapshots
Create:
vxassist –g diskgroup [-b] snapstart origvol
(vxassist –g diskgroup snapwait origvol - use this command to force a wait for the snapshot mirror to finish synchronizing)
vxassist –g diskgroup snapshot [origvol] [snapvol]
Reassociate:
vxassist –g diskgroup snapback snapvol
or
vxassist –g diskgroup –o resyncfromreplica snapback snapvol
Dissociate:
vxassist –g diskgroup snapclear snapvol
Destroy:
vxassist –g diskgroup remove volume snapvol
Snapabort:
To remove a snapshot mirror that has not been detached and moved to a snapshot volume, you use the vxassist snapabort option.
vxassist –g diskgroup snapabort origvol
Displaying Traditional Volume Snapshot Information
vxprint –g diskgroup –ht (or vxprint –htg diskgroup)
Creating And Managing File System Snapshots
Create:
mount –F vxfs –o snapof=origfs[,snapsize=size] destination snap_mount_point
Refresh:
mount –F vxfs –o remount, snapof=origfs[,snapsize=size] destination snap_mount_point
Remove:
umount snap_mount_point
Using a File System Snapshot
After creating a snapshot file system, you can back up the file system from the snapshot while the snapped file system remains online.
To backup a snapshot:
vxdump [options] [snap_mount_point]
To backup the snapshot to tape:
vxdump –cf /dev/rmt/0 /snapmount
To restore the file system from tape:
vxrestore –vx /mount_point
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