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Disaster Recovery (DR) Vs High Availability (HA) Expand / Collapse
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Posted Friday, January 18, 2013 12:27 AM
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hi all,

What is the difference between DR & HA. What are different options available in SQL Server for DR/HA.

The below concepts
1. Clustering.
2. Mirroring.
3. Log shipping.
4. Replication
5. Backup/Restore

Which is related to DR and which is related to HA and why it is DR/HA?

please clarify this very basic questions.

Thanks in advance


Post #1408736
Posted Friday, January 18, 2013 1:03 AM


Old Hand

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High Availability means your database will be available for usage in a very large proportion of the time.
It is measured in percent of uptime or number of nine's, e.g. 99.99%.
Basically it means "your DB and its data will (almost) always be available to users".

Disaster recovery is a process of getting back to "available" state once your db becomes unavailable to users.
That's why DR is tied to HA, it's a process that resoves other part of those nines percentage (e.g. spending 0.01% of time in recovery). Sometimes that DR will be with data loss. There should be defined upfront how much data loss is acceptable.

You need a combination of healthy and redundant hardware and software (not just SQL Server software) to achieve HA and do DR.
Each solution is different from what it protects from (e.g. user dropped table, motherboard failure, storage failure, network failure, OS patching downtime, site failure by e.g. fire or tsunami, ww3, earth exploding or getting eaten by a black hole)

Clustering and mirroring helps you achieve HA. Log shipping will not give you very high availibility, but it will be higher than not having it, and because of time lag there is a data loss. Synchronous mirroring gives you zero data loss only if it is in "synchronized" (not "synchonizing") state. Cluster protects from motherboard failure and patching downtime, but does not protect from storage failure. etc.
Backup is your last resort in data recovery. But mirror, log shipping and replication can also help you to recover your data.


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Post #1408744
Posted Friday, January 18, 2013 3:00 AM


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Here's a good article to help you to get to your answer: SAN Replication vs. Clustering vs. Mirroring Which should be used when? by Denny Cherry.
It also contains an interesting link to an article describing DBMirroring as a DR solution ( for some cases )


Robert L Davis ( @SQLSoldier / blog ) is publishing a great series about Disaster Recovery ( DR ) :

e.g. the common concepts for Writing SLAs for Disaster Recovery


Johan


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Post #1408780
Posted Wednesday, January 23, 2013 4:34 AM
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Thanks for your explanation.

But still i am not getting the exact point of HA vs DR. Can you tell me in which case each of the solution is applied for HA/DR. Like if we take:
Clustering : Is Completely HA.

Mirroring : ?

Log Shipping: ?
...
...


Post #1410471
Posted Wednesday, January 23, 2013 5:08 AM


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build up a solid view !

It's the combination that counts ... if it fits your needs and goals !

http://technet.microsoft.com/en-us/library/hh393576.aspx

http://msdn.microsoft.com/en-us/library/jj215886.aspx

http://sqlblog.com/blogs/kevin_kline/archive/2012/07/26/high-availability-white-papers-and-resources-for-sql-server.aspx


Johan


Don't drive faster than your guardian angel can fly ...
but keeping both feet on the ground won't get you anywhere

- How to post Performance Problems
- How to post data/code to get the best help


- How to prevent a sore throat after hours of presenting ppt ?


"press F1 for solution", "press shift+F1 for urgent solution"


Need a bit of Powershell? How about this

Who am I ? Sometimes this is me but most of the time this is me
Post #1410494
Posted Wednesday, January 23, 2013 5:50 AM


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Do you truly understand what data recovery means? And what HA means?
SQL HA and DR solution (e.g. mirroring or cluster) is not the only thing you need.
They help to achieve HA/DR but often are combined and not alone.
You have to use a combination of various sql and non-sql software and hardware to achieve given HA and DR goals.
Also, there should be a list of what types of failures you plan to protect from.
E.g. user deleted data by mistake - from that type of failure you can recover from e.g. database snapshot.
From disk failure you can recover from backup, mirror, replication, replicated storage.
Disaster could be for example:

- accidental data deletion (that also makes data unavailable to users)
- partial storage corruption
- storage failure (total)
- server failure (e.g. motherboard)
- network failure
- site failure
- multiple sites failure

You can combine cluster with SAN replication, cluster with replication, various network redundancy solutions, storage redundancy solutions, special security (physical and logical), etc.
Each of which can participate in HA or DR or both.
There are also fine details you should know about each solution component and how that affects availibility and data recovery.

So, to bring that all up together into one simple table with e.g. 20 columns x 20 rows
and fill those 400 fields - if that is what you are asking, you will probably not get it.


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