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  • 1.  SQL - VMFS vs RDM?

    Posted Jan 06, 2011 12:01 PM


    I have to configure a new SQL server and use VM for it. The utilization shouldn't be very big. I've been wanderining if I shall use RDM or VMFS. The partition for databases will be 30GB, the partition for the logs - don't know yet - waiting for info from DBA.

    I more keen on VMFS - it seems that it will be easier to manage than RDM, and the performance wise it is not much worse than RDM.

    I'm also going to use a thin format for the databases partition and a thick for logs.

    Do you have any suggestions?


  • 2.  RE: SQL - VMFS vs RDM?

    Posted Jan 06, 2011 12:11 PM

    If you are looking at high data I/O then you can use RDM however i would suggest that using a VMFS is much beneficial.

    Also with RDM's you need to have one LUN per VM

  • 3.  RE: SQL - VMFS vs RDM?

    Posted Jan 06, 2011 01:07 PM

    I think I would keep SQL on a separate LUN anyway - create VMFS and then create drives for System, Data and Logs.

    Would be there any benefit of distributing each partition/drive to a separate LUN rather than having all drives on one LUN - one VMFS partition?


  • 4.  RE: SQL - VMFS vs RDM?

    Posted Jan 06, 2011 06:59 PM

    Really, that depends on your storage subsystem.

    Benefits of having different luns for data/logs is that you are running on different spindles.  And can utilize different raid levels for your data (raid 5) and logs (raid 1).

    That being said, depends on the storage subsystem.  We are an hp shop, and if we carved luns on our MSA, it would make sense, as you assign different physical disks to logical drives, however, when utilizing our eva, all disks are in whats called a disk group, therefore, all io's are hitting all disks no matter the logical drive...

    And again, you have to know the workload of that SQL server.  If you say it isn't heavily utilized, I'd do them all on the same lun to start.  You can always storage vmotion at a later point if performance becomes and issue.

  • 5.  RE: SQL - VMFS vs RDM?

    Posted Jan 07, 2011 08:39 AM

    I will be using HP Lefthand P4300 - according to what I read, the RAID incorporates all disks of the box (RAID5).I think I will put everything on one LUN. Thanks.

  • 6.  RE: SQL - VMFS vs RDM?

    Posted Jan 07, 2011 09:45 AM

    the maximum per LUN for VMFS is only 2Tb-512b. so you will need to carve out multiple LUNS in your case.

  • 7.  RE: SQL - VMFS vs RDM?

    Broadcom Employee
    Posted Jan 06, 2011 01:07 PM

    Hi Marcin,

    Go for VMFS. The I/O performance between RDM and VMFS is almost the same. I would base my decision more on the flexibility. However, when you are planning to use SAN specific features like LUN snapshots you’ll have to use RDM’s. You can find more about this in this doc from VMware (see page 10, Storage topic)

    - hth



  • 8.  RE: SQL - VMFS vs RDM?

    Posted Jan 06, 2011 07:46 PM

    Like some have already stated.  It depends on your underlining storage system.

    It regards to I/O, there isn't much of a differerence between VMFS or RDM..  RDM is marginally faster.   The question is if your SAN has tools for WIndows, SQL, inregards to snaps and backups.

    A Physical RDM will allow a VM to pass SCSI commands to the lun..  In our case we use NetApp.  Netapp has SnapDrive, and Snap Manager for SQL.  This allows use to take snapshots of our SQL at regular intervals and restore quickly if we have too.  Since the storage system talks to the VM it makes life easier when it comes torecovery of data and also looking at the health of the lun.

    If you have no need for any of that, and your database will not grow to be very large, then go with VMFS.  Otherwise use RDM.