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Oli_L
Enthusiast
Enthusiast

SAN Disk configuration to run VMs

We are just about to deploy a new infotrend 18.6TB SAN storage unit and are deciding how to go about the RAID configuration

We are to run our document management servers and our new exchange servers on this unit.

Could anyone recommend the best practises to run VMs on SAN storage.

Questions are

What RAID set do VMs need to run on

Do you need to keep the VMs on different underlying disks for load balancing?

Could I run all my VMs and DBs on say RAID5 or would it be best to run on different RAID sets, ie 1 for the VMs and 5 for SQL server DBs

Should I partition the disks for my logs files, page files and DBs so they all sit on different partitions

Do I have to account for the VMs which are now sitting on SAN disks compared to once when they sat on physical disks?

Thanks for any input people can give

Oli

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Michelle_Laveri
Virtuoso
Virtuoso

OK... quite a lot of questions here! Smiley Wink

Couple of things... Raid levels do affect i/o. A lot of people like RAID10 because you are protected from 2xdisk failures (usually in different shelves)...

It is possible to give any VM more than one disk (boot disk, log disk and data disks), and store these on different LUNs with different RAID levels appropriate to your fault-tolernece/performance requirements...

WARNING: The above configuration is incompatiable with the VMware "snapshots" feature. Its a bug. Snapshots are used for backup processes so are important...

Other guides lines: VMware recommend no more that 16 esx hosts pointing to the same LUN/VMFS volumes similtanously. They don't really offer much guidence on LUN sizes. But generally a few number of large LUNs is less work for your SAN than a large number of small LUNs. It also means you need to speak to storage people less frequently...

Regards

Mike

Regards
Michelle Laverick
@m_laverick
http://www.michellelaverick.com
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Oli_L
Enthusiast
Enthusiast

Thanks Mike,

I have a concern that I will be placing too many production servers in the same LUN running on the same set of underlying disks.

The SAN guys are slicing up some new storage this weekend and plan to have 4 sets of RAID5. They will create 2 pools - each pool will consist of 2 RAID5 sets. When I ask for a LUN, the SAN administrator creates the LUN from a pool - therefore I can never really tell where physically on which set the VMs will reside. I need to load balance my VMs - say put the OS of one LUN, Pool1 and the LOGs on the other LUN Pool2 and vice versa for another VM - my concern is having LOGs OS and DBs all of the LUNs regardless of how I load balance.

Just trying to get another opinion so be glad to hear your thoughts.

Thanks v much!

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mpverr
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Enthusiast

Whatever you do make sure you have the LOG files on a separate LUN than the database files. We are using EVA5000 for our Exchange 2003 implementation and separated the database, transaction logs and smtp traffic all on separate luns. We were very focused on our configuration for maximum IOPS. The current implementation we have is overkill (averaging only 0.4 IOPS per host with an average of 4000 users per host). With that said, you need to consider for exchange performance what your clients are doing. If they are using CACHED mode then you will have far less IOPS load on the SAN than if they are not in cached mode. Typically log files are placed on Raid 1+0 LUNS since logs are sequential and not dynamically read/written to. You didn't mention the size of your organization so a lot of this is guess work of course. You could easily get away with Raid 5 on the databases if you have lots of spindles in that disk group to maximize the available IOPS. For logs - try to setup a a good size LUN (with at least 24 disks - again depending on your organization size) for logs. You could add both exchange and SQL logs onto that same LUN (again size of data matters). One thing to keep in mind is your backups. If you are not going to leverage VCB then you will backup over the LAN (not good). If you are giong to backup your LUN at the san level then you only want Exchange in that Lun. If you are going to use VCB then remember that VCB will quience the vmdk but not the database within the VMDK - so you need to really ensure you understand how all that works.

Hope this helps rather than confuses.

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mpverr
Enthusiast
Enthusiast

Ugh - one other thing.

ESX3 went a long way in improving the SCSI locks for multiple access to a VMFS-3 LUN than ESX2 with a VMFS-2 Lun. But keep in mind there is still disk contention. You need to be mindful in properly balancing low and high disk I/O type virtual machines and the VMFS-3 LUNS you will place this stuff on.

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