Asset ID:	1-71-1017411.1
Update Date:	2011-08-11
Keywords:

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Solution Type  Technical Instruction Sure

Solution  1017411.1 :   VTL - Why is write cache mirroring disabled?  


PreviouslyPublishedAs
228498

Applies to:

Sun StorageTek VTL Plus Storage Appliance - Version: 1.0 - Build 1323 to 2.0 - Build 1656   [Release: 1.0 to 2.0]
Sun StorageTek VTL Prime System - Version: 1.0 - Build 1813 to 1.1 - Build 2076   [Release: 1.0 to 1.0]
Sun StorageTek VTL Storage Appliance - Version: 4.0 - Build 1221 and later    [Release: 4.0 and later]
All Platforms
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***Checked for relevance on 05-08-2011*** (dd-mm-yyyy)

Goal

To describe why write cache mirroring (WCM) is disabled in certain version/revisions of VTL.

Solution

For versions of VTL without the "Get Well Plan" the following applied:
OS Disk support always recommends write cache with mirroring, as this is the safest data protection. However, there is the performance impact, and VTL engineering worked with OS Disk engineering and backup vendors to perform a risk assessment before VTL was released.
Their conclusion was that for the "typical" customer backing up data, the performance impact outweighed the low risk of data corruption and the default is to disable write cache mirroring for all VTL implementations. That being said, this is a configurable setting and depending on each individual customer's data requirements, this can be enabled.

The next question will be, under what conditions would data loss occur with write cache mirroring disabled, response from engineering:
The risk to data is small - basically a controller has to quit responding while data is being written to it, so whatever it has in cache gets lost. A normal AVT won't lose the cache data, since the first controller will feed the second the cache before it turns over the volume in a transfer.

For version of the VTL that have updated to the "Get Well Plan":
WCM was enabled in the VTL with the installation of the Get Well Plan. It was found that customers preferred the extra protection that it provided in regards to the data.

Conclusion:
If a VTL Plus or Prime has not had a Get Well Plan applied it is HIGHLY recommended for data protection. Please work with Oracle Field Support to get this installed.

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