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I am implementing an SQL failover strategy for ms-sql 2k8, and am thinking of going with database mirroring. I've read that it's not intended for a large number of databases - however the article did not define 'large number'. I'd be looking to mirror around 70 databases.

Are there are limits to how many databases can/should be mirrored - is it solely dependent on bandwidth, or is it the effort of setting the mirroring up per database, or are there other issues?

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3 Answers 3

Bandwidth will definitely be one consideration. Another will be the server resources which are required to run database mirroring for that many databases. Database mirroring will be contently shipping transactions from the active server to the mirror, this will require a small amount of resources per database. Multiplied by 70 databases that little bit will add up.

With 70 database mirroring sessions running, you'll have 70 threads running in the background which will probably add quite a bit of context switching to the system.

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@MrDenny: Out of interest, what other options does MCW have, re: fail over? What about simple RAID-1 (or any RAID that offers mirroring). I know that doesn't answer the problem if the building is in trouble (fire/nuke/etc) .. so offsite works for that. Just curious :) –  Pure.Krome Jul 2 '09 at 8:55
    
A SQL Server should always be setup using RAID of some sort, either RAID 1, RAID 5, RAID 6, or RAID 10 depending on what's needed for this particular setup. As far as local HA @MCW can look into log shipping, replication or clustering. Clustering is the most effective as it provides near instant failover, however it requires shared storage which makes it very expensive to setup. It also moves all the databases from one node to the other as a group so interdatabase dependencies aren't an issue which they can be when using database mirroring. –  mrdenny Jul 2 '09 at 9:00

On a 32bit machine there is a limit of 10 databases.

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Not true. There is no "hard" limit enforced by SQL server

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