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Microsoft SQL Server 2012 - Seperate drives for Logs and DB?

Posted on 2013-12-05
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Last Modified: 2016-11-23
Hi,

I'm just after peoples thoughts really.

All our MS SQL Servers are virtual machines.
Currently the VM's have two LUNS presented to them (iSCSI via Passthrough disks), one for SQL databases and the other for SQL logs
The LUNS all reside on the same group of Dell Equallogic iSCSI Arrays

Considering that the database and log drives both exist on the same storage arrays and on the same disks, is it worth having separate LUNS for databases and logs?

Performance is great with the current setup, so I don't see combining logs and databases on a single LUN will affect that.

We backup every 30 mins using Microsoft DPM

Any thoughts?
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Question by:AJB_KRC
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Andrej Pirman earned 2000 total points
ID: 39697969
Your thoughts are correct.
Since it is same storage behind, the performance would not be better if you separate logs and DB on different LUNs. Maybe it is even better because using 1 LUN on iSCSI means iSCSI TCP encapsulation occures once, and for 2 LUNs twice, but I think it is unmeasured difference.
The gain would be if you would have LOCAL SSD disks, for example, in RAID1 config on server and use them for LOGS (as locally attached storage). RAID controller cache would help a lot, but that depends on your SQL Read/Write ratio in your usage scenario. IF you use, for example, 70/30 R/W average, then still 70% of performance would depend on reading from database (RAM and cache optimization helps a lot), and 30% are writes to file system (actually to RAID controller's cache).
So with local disks, this WRITE operation would happen faster and since MSSQL considers query done after it is written to LOG file (writing to DB happens afterwards and is not essential for performance!), this would improve some things.

So, in your current config I only see possible improvements in RAM and cache optimization for VM and of course, I hope you are NOT using MSSQL Express, rite? Express has 1GB RAM usage limit :)
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by:AJB_KRC
ID: 39720009
Thanks Labsy - we've started moving to single LUN's so we'll see how it goes!
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