August 20, 2009 at 9:34 am
After reading through various books, I not seeing why a cluster would be better than mirroring for HA; and its more expensive and less flexible. It seems to need identical machines, Enterprise windows and sql and you still have a single point of failure on the drives unless you do something extra there.
What am I missing?
tks.
August 20, 2009 at 9:42 am
kevinH (8/20/2009)
you still have a single point of failure on the drives unless you do something extra there.
You're right but usuallly the shared disks array are fault tolerant(SAN/NAS) as they're fairly redundant.
In mirroring, there is slight possibility of data loss if you're using asynchronous mode and the log of the primary gets corrupted. Also, in synchronous mode, your performance might suffer a hit as it'd wait for the transactions to commit on the mirror. In clustering you donot have to face such issues.
EDIT - Fixed the quote
August 20, 2009 at 9:43 am
what if you had multiple application databases that needed to be failed over at the same time? i.e. interdependencies? In that case the inflexibility of clustering would be a good thing.
I suppose MS could address DB mirroring's shortcoming in that area by allowing some sort of triggering mechanisms that would fire upon a DB mirroring failover (like allowing you to code manual failovers of all other mirrored DBs on the server)...
or what if you had non-SQL Server DB Engine components that needed to failover as well? (IIS, SSIS, Reporting)Services).
October 10, 2009 at 2:48 pm
kevinH-1006814 (8/20/2009)
After reading through various books, I not seeing why a cluster would be better than mirroring for HA; and its more expensive and less flexible. It seems to need identical machines, Enterprise windows and sql and you still have a single point of failure on the drives unless you do something extra there.What am I missing?
tks.
SQL cluster is the preferred solution for HA. It works at SQL instance layer, Mirroring works at database layer. Automatic fail-over is tricky on mirroring, not automatic is your application is not using the right driver; SQL cluster, in theory, is, because you are using one unique virtual cluster name.
I personally prefer Cluster as HA. Is a more mature product and can provides load balance in active/active solutions. If I recall well, Mirroring has no official Microsoft support in case of problems if used on production environments.
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