EMC Symmetrix Timefinder Mirror operations

We discussed about Timefinder operations overview in my last post. Today we will check more about Timefinder Mirror operations.

Timefinder\mirror creates true mirror images of its source volume.It uses a special attribute device called Business Continuity Volume (BCV). Data will be copied from the standard device(Standard devices are Symmetrix disks that are configured for normal Symmetrix operations under a desired protection method such as RAID 1, RAID-S, RAID 5, RAID 6, and SRDF) to the BCV devices when we establish the BCV operations.As I told earlier while establishing, BCV device will act as a mirror of the standard volume and can involve in regular i/o process from host. BCV copies can be used for backup, restore, decision support, and applications testing. Each BCV device has its own host address, and is configured as a stand-alone Symmetrix device.

BCV Devices:

A BCV device is a standard Symmetrix device which has a special attributes. It can be accessed  independently and can  support host applications and processes A BCV device can be unprotected, Mirrored (RAID1), RAID-S, RAID5, RAID6, and SRDF R1 or R2.Within the Symmetrix, each device is represented by four mirror positions: M1, M2, M3, and M4 and when BCV device is established to a standard device, it will occupy one of the standard mirror position.I have explained about the mirror positions in my previous post.

Note :

1. RAID-5 BCVs where introduced in Enginuity 5671. It was not available in 5670, even though RAID5 was available

2. RAID-6 BCVs where introduced in Enginuity 5772.

Starting from 5671,  whevever TimeFinder\mirror  encounters a RAID 5 BCV  device, it automatically maps TimeFinder\Mirror command to its corresponding  TimeFinder\Clone. This is called Timefinder\Clone emulation mode.

In all Vmax models (Enginuity 5874 and higher), TimeFinder\Mirror uses Clone Emulation mode for all operations.

Common BCV Operations:

The common BCV operations are below:1. Establish2.Split3.RestoreWe will check each in detail.As I told earlier if BCV device is a RAID-5 device, symmetrix will use timefinder\clone emulation mode.  So in this article we are taking example of a mirrored BCV and standard device.

Timefinder\Mirror Full Establish

Establish is one of the common mirror operations and below picture will give an idea about this process

During the establish operations, the BCV devices will act as a mirror of the standard device and data will copy from the BCV device to the standard device.Establish operation can be full or incremental. We will check the incremental establish at later part of this article.

If the BCV device is establishing to the standard device for the first time, full volume synchronizing is required.During this process entire data from the standard device will be copied to the BCV device.During the establish process, BCV devices will be not ready to host and if the BCV device is mirrored, the second mirror (BCV M2) will be detached and all tracks of the M1 are marked as invalid.A track by track synchronization takes place as a background task at a lower priority than front-end activity. Once the data is fully synchronized, the BCV continues to be a mirror of the standard device until it is split. One advantage of TimeFinder/Mirror is the added availability that the extra mirror provides. If the M1 and or M2 of the Standard device  become unavailable, the BCV can be used to service host read write request to the standard.

Timefinder\Mirror Split

Below picture will give you a logical view of BCV split operations

As I told earlier,  Once fully synchronized, the BCV continues to be a mirror of the standard device until it is split.During the BCV split operation, BCV will split from a standard device and it  becomes an independent device and is available for host access. If the BCV is mirror protected, the M1 is used to incrementally resynchronize the M2.

Timefinder\Mirror Incremental Establish

During Incremental establish process,tracks that are changed on both the Standard and BCV devices are copied from Standard to BCV.To enable incremental operations, TimeFinder/Mirror keeps track of changes to the Standard and the BCV following a split. A full synchronization is always required initially, but subsequent establishes by default will be incremental, where only the data that has changed since it was split is resynchronized. Just as with the full establish, by default, the BCV M2 mirror (if there is one) is detached.

Timefinder\Mirror Restore

During a restore, the BCV becomes a mirror to the Standard device and becomes not ready for host access.Similar to establish operation, during a restore, the BCV M2 (if there is one) is detached. The BCV M1 become the “valid mirror” and is used to resynchronize the Standard Device. The updated data from the BCV is instantly available to the host attached to the Standard device while the resynchronization occurs in background. New changes to the Standard are also propagated to the BCV. When a Restore completes the BCV can be split of for use or remain attached as a mirror.

Timefinder\Mirror Protected Restore

As discussed above during a normal restore operation, changes to the standard devices are also copied to the BCV.If you want to protect BCV data during the restore operations, you can use this option.In protected restore, the BCV image will be  protected so that changes to the standard are not copied to the BCV. This is useful in situations where the BCV contains the “Golden Copy” of the data and must be preserved. Take an example of where there is corruption to the standard device. The data on the BCV can be used to restore the production data and the application can be restarted while synchronization takes place in background. Because the BCV is a mirror of the standard, if the corruption occurs again, both the production data and the BCV data would be lost. The protected restore option allows one way synchronization from the BCV to the standard.

Timefinder\Mirror Protected Establish

Protected Establish is similar to a normal Establish except the M2 mirror is not detached and both mirrors are synchronized simultaneously and it will give additional level of availability.

Timefinder\Mirror Reverse Split

Reverse Split provides a way of rolling back the data image of the BCV to the state prior to establish. This is another way of preserving the BCV “Golden Copy” While a BCV is established, it becomes another mirror of the Standard Device. If there is data corruption, it will impact both the Standard and BCV devices. By performing a Reverse split, the BCV M2 mirror which was detached while it was Established, and thus protected from corruption, is used to synchronize the BCV M1, effectively rolling back the data image to where it was prior to the establish.

Timefinder\Mirror Concurrent BCVs

With Concurrent BCVs, two BCVs can be established to the same standard device at the same time. After synchronization, they can be split together to have two copies of the same data or split separately to have two different point-in-time views of the data.

This series will continue in next post. Hope this was helpful.

Happy learning !!