What BCV and 2-Way-Mirror are and how to use them

Question

Hello,

These could appear to be quite basic questions, however taking into account that EMC does its best to cover its mechanisms with a shroud of magic, such questions become quite understandable.

BCV is not RAID-1, right? Is it kind of a local synchronous replication mechanism from a source device (STD) onto a target device (BCV)?

What happens if a source device fails? Does BCV automatically replace anyhow the broken STD with itself? If not, what do we need to do with the BCV to replace the broken STD with it? Or do we need to restore a BCV onto another STD in order to get a properly functioning STD?

Could you bring any handy examples of using BCV?

Why does a BCV device still have a logical BCV name (BCV001, for example) even after the split operation has been performed? Isn't it already a regular device and should be treated as a regular device?

2-Way-Mirror is RAID-1 in terms of EMC? How data on a 2-Way-Mirror is redistributed across physical drives? Couldn't it happen that one physical drive stores blocks from both parts of our mirror (the same blocks needed to provide data redundancy)?

What is 2-Way-BCV-Mir?

Can we use 2-Way-Mirrors and BCVs in a completely thin provisioned environment? Does Enginuity have mechanisms ensuring that mirrored blocks of our hypothetical 2-Way-Mir+TDEV and BCV+TDEV do not reside on the same physical drive? If so, how it can be useful and what are the steps of creating such devices?

Thanks in advance.

Quincy561 · Answer

Before VMAX, a BCV could become a true mirror of a volume, and would occupy one of the 4 mirror positions of the source volumes.

With the introduction of VMAX, we dropped the support for true BCVs in favor of TF Clones.

In a VP (thin) environment, the devices that are the source and the target don't have protection directly, the pools they are allocation on have the protection, and in a FAST VP configuration, they could be on more than one tier, and all could have different protections.

Symmetrix will not allow devices that protect each other on the drives to be on the same disk, or directors where any single component failure will stop access to the volume.

Kult · Answer

Hey Quincy56, Thanks for your reply, however it gives quite general answers. Is there any way for you to help me with each question I asked? Thanks indeed.

Kult · Answer

Thanks for mentioning this document, it's exactly what I currently read.
There are such words in the chapter 5 "Performing TimeFinder/Mirror Operations" (page 108):

Once a BCV device is established as a mirror of a standard device, those two devices together are referred to as a BCV pair. The pair consists of two types of mirrors: the standard device mirror(s) and the BCV mirror.

The standard device mirrors contain copies of the data contained in their associated standard devices. There can be up to three standard device mirrors (M1, M2, M3).

A BCV mirror is a standard device mirror. It can be a two-way mirror (M1, M2) that is assigned upon creation of the BCV pair.

I can't get the idea. How come the source device becomes a mirror and contains copies of the data contained in their associated standard device? I thought as a replication/clone source it should be just a source with original blocks of data (not some sort of copies of the original blocks).

Also why a 1-Way-Mirror is called "Unprotected"? For me it looks like data is replicated from one source (STD) onto one target (BCV). Appears to be quite protected.

3-Way-Mirror is one STD and 3 BCV devs, right?

PedalHarder · Answer

BCV is an attribute placed on a device. So it could be RAID1 or RAID 5, or even unprotected. As quincy states, the BCV mirror technology does not apply to VMAX. Since you are talking TDEV, TimeFiinder Mirror does not apply to your situation. If you are still interested in the technology, you will find some good documentation in:

The EMC Solutions Enabler Symmetrix Timefinder Family CLI Product guide.

PedalHarder · Answer

When a BCV Establish is performed in a Timefinder mirror operation, a mirror position of the BCV moves into one of the available mirrors of the standard device. Data will then flow to the BCV mirror from the standard until they are in synch. A Timefinder SPLIT will remove the BCV mirror from the STD device.

If the BCV was created as unprotected,if there is a drive failure where the BCV mirror lives (when the BCV is in SPLIT state), you just lost your data.

If the BCV was created as 2-way-mir though, once the SPLIT is done, the BCV then starts to mirror the mirror position that was attached to the STD, over to the second BCV mirror. When this is complete, you have a protected BCV. i.e. a single drive failure will not cause you to loose your BCV image.

While devices have 4 mirror positions available, not every mirror position needs to be occupied. Any 3 of the 4 mirror positions can be used as 'floating mirrors'. That is, they can be occupied dynamically, such as in a BCV establish. This is why it talks about 3 mirror positions in the graphic.

Quincy561 · Answer

TDEVs are cache only devices and have no protection.  The pool they are allocated on has protection.

Quincy561 · Answer

In your picture, a 3 way mirror device is simply a 3 way mirror device. Not one mirror and 2 BCV devices.

Are you working with a VMAX or a DMX?

None of this applies to VMAX, since it never supported BCV mirrors.

Kult · Answer

Very exhaustive answer. Thanks indeed.

VMAX

What BCV and 2-Way-Mirror are and how to use them

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