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The Best High Availability


The Best High Availability

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Steve Jones
Steve Jones
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Comments posted to this topic are about the item The Best High Availability

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Jason Miller-476791
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We have medium-high availability. Our db servers run fibre channel over to our EVA, which is a single-point of failure. (I know it's got redundant this and that.) If a box goes belly up, we created a script that transfers all of the storage from our production box to a test box. (All db servers are identical.) Total down time should be <5 minutes. That includes detecting the problem.

Our investment accounting system runs on a cluster, serviced by the same EVA. The amusing thing is, if a cluster node goes down and the db needs to migrate to the new node, SQL Server will stop for a couple of seconds (as expected), the problem is... The IA system is so well written, that if SQL server goes down, it'll puke and die, but not before corrupting the databases that support it. I could go into detail as to why, but let's just say they (the vendor) do not believe in such trivial things as transactional integrity, indexing schemes, data validation, stored procedures, system views (why bother when you can update the system tables directly?)... You get the idea.


I like the idea of our current environment, along with the addition of log shipping down to our DR site. w00t We're on a 15 minute lag, so for us it's not bad. After all, we're not Wal-mart, or Amazon...



(Steve, nice new pic... Shamed me into changing mine.)

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Jason Miller
jay-h
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Our old 2000 server was clustered and was problematic at best due especially due to the shared drive requirement. Drive hiccups on the RAID box were our most common problem.

For 2005 we implemented mirroring which so far has been satisfactory. Since the drives are on two physically separate systems, we can even bring down the drives for maintenance (firmware etc) while keeping the database up.

...

-- FORTRAN manual for Xerox Computers --
webrunner
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I have a general question - can virtualization play a part in HA? In other words, is it possible to have a setup where a SQL cluster is not needed if virtual servers are used?

If so, are there sites or articles someone can point me to for discussions and pointers?
If not, are there still ways to make use of virtualization of SQL servers?

Thanks for any advice.

- webrunner

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Jason Miller-476791
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webrunner (4/28/2009)
are there still ways to make use of virtualization of SQL servers?



We have a couple of VM servers that house production SQL environments. Our corporate accounting system, a bug tracker, and an on-site inventory management system are all on VM SQL... That works well for us for they are very low usage/volumes. The corporate accounting system is the biggest with a little over 1GB...


That being said, I am familiar with a company that is running a 660GB email tracking system on a virtualized SQL box. Its performance is pretty abysmal. I wouldn't recommend any system more complicated than a recipe look-up on a virtualized box.

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Steve Jones
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Virtualization definitely can, and I wouldn't be surprised to see more of this.

You can cluster two virtual servers as long as they can see the same shared disk. So the underlying Windows host(s) need to be able to share a disk somehow.

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DPhillips-731960
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It is OK to VM the SQL Server instances but I wouldn't vm the data with it. The nodes need to see the same disks (as Steve has said).

Still VM is some order slower than a dedicated box, as everything is being shared from network connections to CPU cycles. Some intelligent analysis and testing would be in order. I wouldn't use it for HA systems unless those systems are strictly lookup systems with DBs that can reside in memory, or are extremely focused and minimal in overhead.
DPhillips-731960
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Just thought of another way to put it... I personally view VM technology as optimum when the O/S is merely an interface.

For example, it is great in dev-test environments, where testing against multiple systems and platforms is needed.

It is great for small web-hosting instances too, where each instance can have 'root-like" access to their own instance.

It is still not great for databases (especially large implementations), nor integration, report, and analysis servers.
David Reed-223505
David Reed-223505
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I'm still wondering which HA scenarios clusters can't solve, Steve. Please elaborate... Offsite DR doesn't quite fit as a typical HA scenario, but it's still doable with dark fiber and the right SAN gear.

The "best" HA will be a layered approach anyway. Mirroring or clustering for "immediate" HA (such as hardware hiccoughs) combined with log shipping for warm standby at a secondary NOC (to deal with minor disasters, internet service outages, failure to pay the light bill) and offsite backups on removeable SSDs or DVDs in another region or country for recovery from true disasters.
Rick Romack
Rick Romack
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We implement HA in various ways here. We have some clusters of SQL 2005 on Windows 2003 which means we cannot geographically separate the servers. We use Log Shipping on some databases and Database Mirroring on others. In one particular case we mirror a database that is on a cluster to a geographically separate site. In some instances we use the old tried and true backup and restore.

When a business unit presents the need to store data within SQL I present them with the options trying to give them an over view of the good and the bad of each option. Surprisingly most requests end up being that "Last nights backup is good enough for us."

One other thing I see a lot of is implementing an HA plan and then never testing it. Testing an HA has to be done on a regular basis or you may and probably will end up with unrecoverable data during a disaster.

Rick Romack
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