March 6, 2008 at 6:11 pm
Hello,
I'm fairly new to sql and wondering if someone can share some light on why the disk queue length is so high (568.191). My customer is reporting the system is performing very poorly. This volume is a SAN drive 6+1 Raid 5.
March 6, 2008 at 7:24 pm
Is it sustained or bursts?
It will be high because something is doing a lot of reads and/or writes. There are perfmon counters for each, check them for a start. Then run SQL Profiler to see what your queries are doing, most importantly reads & writes.
You can check/update your index statistics.
Start with the worst performing queries and check the execution plans. If there are lots of table scans going on, a few well-placed indexes will help. And/or better written queries.
You could also look at putting your db data & log files on separate physical disks.
Basically there's a number of things that can improve performance. It depends on what's going on and what your current configuration is.
Of course, I am also assuming that SQL Server is the culprit (given the forum). Is there anything else running on the server (there shouldn't be)?
How big is your database? What is it used for - what's the balance of reads & writes?
Lots of questions to be asked.
MARCUS. Why dost thou laugh? It fits not with this hour.
TITUS. Why, I have not another tear to shed;
--Titus Andronicus, William Shakespeare
March 6, 2008 at 7:30 pm
Your average disk queue length is a serious cause for concern. There is a huge i/o bottleneck. I am not at all surprised that your customer is complaining. In fact, I am surprised that your customer can do anything at all (except for complaining). I would expect that timeouts being experienced by your customer quite often.
You really need to look into why there are so many i/o requests.
My first thought is that there are indexes missing. I would suggest that you run Profiler and identify queries that are doing a significant number of reads and check what indexes may help these queries. The "Database Engine Tuning Adviser" in Management Studio should help to identify indexes that may be missing.
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