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Thread: ESX disk write caching enabled or not

  1. #1
    Join Date
    Oct 2010

    ESX disk write caching enabled or not

    I have a double which is needs to be solved because it is very important for my Storage Area Network. Has anyone heard about the disk write caching or something similar words? The thing is how can I verify this if it is enabled? The LUN also called as Logic Unit Number is set to enable on SAN. But I am feeling some bad performance issues. The issue like Read = 349MB/s, Write = 90MB/s. Do you have any clue about this thing? I didn’t get any result yet. So what do you think about it?

  2. #2
    Join Date
    Feb 2010

    Re: ESX disk write caching enabled or not

    I cannot be able to solve with this limited information. You will need to get the IOPS and research them what is wrong with those. For IOPS you can use good tool called Iometer. It is an input/output subsystem measurement and characterization gizmo for distinct and clustered systems. It is normally used as a standard and troubleshooting tool and can be easily configured. With this you can replicate the behavior of the many popular applications. Using this tool see and not down what kind of IOPS you are getting. Generally we don’t touch cache on SAN because that is all based on the controller.

  3. #3
    Join Date
    Feb 2010

    Re: ESX disk write caching enabled or not

    If you are running disk performance metric tools inside of a vm using vmfs3 on a ds3400 then you will get the random numbers which are really confusing. You won’t get the current number. You are unable to get the accurate or perfect performance test on SAN from a inside of a VM. You might have known that there are two file systems for each VM and in this case NTFS on top of VMFS3 and the host in SAN is arbitrating all of the Input/Output and other resources.

  4. #4
    Join Date
    Feb 2010

    Re: ESX disk write caching enabled or not

    I will recap you one by one what is really going on. SAN write performance blows away. In a VM or directly attached otherwise RAID5 5 disks or RAID0 2 disks. The actual problem is that READ performance is more or less brilliant and the write speeds are at BEST 1/2 read speeds even w/RAID0.Otherwise you can test scored by using server other than VN on RAID5 5 disk LUN by disabling write caching on the LUN. As your reading concern I think it is however related to write caching. The other thing is disabling write caching kills IOPS. Make a note that SAN don’t perform like this.

  5. #5
    Join Date
    Feb 2010

    Re: ESX disk write caching enabled or not

    We use the standard VMware method to measure the performance of a VM and of the SAN. For this we use specific ICF for iometer. The settings that we use bypass most of the filesystem limitations and enable us to acquire the pretty pure performance, more than ever when paired with a perf. So you will need to run the test again and also run a vm-support -s from the ESX host at the same time

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