virtual drives on SAN

I am reviewing some redundancy based checks for a database application (MSSQL) but I wanted to gather your view as to whether these best practices are more geared towards physical servers and opposed virtual servers running on a SAN.

The best practice suggests separating database and log files and backups onto different drives, indicating if one drive dies then the other should be ok and it may not be a disaster.

But for drives on a VM hosted on a SAN - can individual drives on the VM still fail as they could in old physical drives. And by fail I mean can they fail in isolation, or would it be more likely the whole VM would fail as all the "virtual" drives on the VM are on the same hardware (SAN). If virtual drives can still die/fail in isolation even though they arent unique hardware as they would have been on an old physical server, can you give some pointers on how/common reasons they would fail/crash/whatever?
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pma111Asked:
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Andrew Hancock (VMware vExpert / EE MVE^2)VMware and Virtualization ConsultantCommented:
There was a time that the statement you have made was applicable to Physical Servers and Direct Attached Storage. Because you had limited numbers of disks, and DBAs created Logs on RAID 1 Mirror (2 disks), DB on RAID 1 MIrror (2 disks).

It was done for performance, and disaster recover, because if logs were on separate disk to db, then you could recover transactions, and roll back etc

also logs could fill up the disk, so seperate disk LUNs were created.

However, rollout the SAN, and modern SANs, have many disks and shelves, which are all RAID-RP, RAID 6

and the disk presented for SQL, is either a datastore/VMDK or VHD, which is cut from the SAN on multiple spindles....

BUT, DBAs still like to create seperate disks for DB and Logs. (performance is likely to be the same as a single disk!)

Generally, I think if your SAN failed, ALL disks/LUNs and datastores would be unavailable...

However, there are many different SANs, and it's possible a LUN could fail, and you could be using two different LUNs for your SQL server.

So the example you give is still possible.
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