SAN Drive configuration for best performance
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I've got a slightly different perspective on the SANs. I work in a large data center, managing DB2, Oracle and SQL across local and SAN storage. Some things I've seen or been recommended:
1. Boot everything on the SAN - has good points and bad. If you have some hardward failure, another box can be up quickly since you don't rebuild the OS. This has come in handy. On the other hand, we don't need to very often
2. Different SANs perform differently. We have an HP EVA (Compaq) and the recommendation there is to RAID 7 (I think?) the entire frame and carve your LUNs out of this. I wasn't thrilled with this from the db perspective by it hasn't caused any issues with a few SQL Server on it.
3. The EMC was similar and our new HDS is more like we have groups of RAID 5 disks (5 I believe) that are used to carve out 10, 20 or 100GB LUNs. This is from the SAN group, not sure if it's an HP recommendation. However both the EMC and HDS perform similarly as we have migrated systems. The EMC is like 3 years old, HDS new in 2003.
4. Fro Oracle, the recommendation is a large SAN mount point with all data and logs on the SAN. DB2 there is some debate, not sure about SQL. However the seralization brought up above is a good point. You want multiple connectivty (for failover), but also to spread loads so you can reduce the serialization.
5. We've been told on the HDS there is no difference between RAID 5 and RAID 1. I spoke with HP and read some whitepapers that basically explain this away because of the large cache on the SAN, something like 10GB. I guess it makes sense, but if you are really busy and don't forget this cache is shared with file servers, email server, whatever else you've got on there, then you still could overwhelm it. I'm skeptical, but that's the current argument.
6. Most of the issues I see are either OS tuning issues with the SAN, bad drivers (or old) or app issues. I'm not sure that any performance issues you see, unless you are really stressing the system, are going to be the SAN. This is a fairly mature technology that rocks. I still like local disks, or at least my "own" SAN disks, but so far it's worked well.
7. Lastly, there's a great writeup on %disk time and how it's not accurate. %idle time is much better counter to use.
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