April 19, 2012 at 6:06 am
Hi all
I am a bit hazy on this area so sorry if what I am asking makes little sense. I was asking our SAN engineer why we had several different volumes for log files even though they were writing to the same RAID array under the hood.
He explained it was because of the way the HBA works and that you get queue limitations in terms of the pipe that writes data through the HBA to the SAN, and by having more volumes you can parallel write different log files.
Does this ring true with people? We are using EMC as our provider but any info and any experienced thoughts are appreciated.
Further to this our new SAN is going to be tiered and not RAID as previously, so does the same principle apply? When I say tiered we have 3 levels of storage FAST, Standard, and SATA. The SAN controls what data is on what speed storage dependant on its use. From a DBA point of view does anyone have any thoughts on what impact this has on our role as DBAs?
Thanks
April 19, 2012 at 9:00 am
This is impossible to answer without know the make/model of both the SAN and HBA's (including firmware and driver versions), multipathing software.
Also, you're pretty much stuck believing what he tells you.
You best bet when conversing with SAN admins is to use SQLIO and do some benchmarking. Brent Ozar has some excellent posts on this topic.
http://www.brentozar.com/archive/2008/09/finding-your-san-bottlenecks-with-sqlio/
For logs, you want to test 1k, 2k, 4k and 8k sequential writes.
1. Try different combinations of threads and outstanding requests, attempt to get the highest IOPS and mb/s possible.
2. Do this on each LUN individually, then edit the config to use multiple data files, 1 per lun. Test again and record results.
3. Present results to the SAN admin.
You either validate what he says, or, you ask him why you're not seeing the benefits he claims are there.
This will hammer the SAN fairly hard, so perform during a time of low activity.
Good Luck! SAN admins are notorious for both not understanding SQL Server IO patterns and not listening to advice on how to configure their SAN.
April 19, 2012 at 2:52 pm
Kwisatz78 (4/19/2012)
by having more volumes you can parallel write different log files.
Can you get the admin to clarify exactly what they mean by this?
SpringTownDBA (4/19/2012)
3. Present results to the SAN admin.You either validate what he says, or, you ask him why you're not seeing the benefits he claims are there.
This will hammer the SAN fairly hard, so perform during a time of low activity.
Good Luck! SAN admins are notorious for both not understanding SQL Server IO patterns and not listening to advice on how to configure their SAN.
Make sure to work with the SAN admin, they'll be a whole lot pissed if you run SQLIO without telling them and put their nice shiny SAN on its backside 😉
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"Ya can't make an omelette without breaking just a few eggs" 😉
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