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"Enable Write Caching on the device" option in Disk Managment

    Question

  • Hello,

    I have a question on the "Enable Write Caching on the device" option on the PRoperties>polices Tab of a disk in our Dell R510 Server.  We have hardware RAID on this server, 3 disks setup in a RAID 5 array.   With Open Manage RAID management installed on the server, we have "Write-caching" disabled.  

    I'm curious what the difference between the RAID "write-caching" settings are compared to the Windows Disk Management settings for Write Caching.  Are the mutually exclusive?  How do they work together when one is disabled, while the other is enabled?  What if they are both enabled?    How does windows interact with a RAID level write-cache setting?  

    Any input on this would be much appreciated.   

    Thanks!

    Tuesday, August 23, 2011 9:55 PM

Answers

  • DELL PERC H700 and H800 Technical Guide 20
    4.10 Virtual Disk Write Cache Policies
    The write cache policy of a virtual disk determines how the controller handles writes to that virtual disk. Write-Back and Write-Through are the two write cache policies and can be set on virtual disks individually.
    All RAID volumes will be presented as Write-Through (WT) to the operating system (Windows and Linux) independent of the actual write cache policy of the virtual disk. The PERC cards manage the data in cache independently of the operating system or any applications. You can use OpenManage or the BIOS configuration utility to view and manage virtual disk cache settings.
    In Write-Through caching, the controller sends a data-transfer completion signal to the host system when the disk subsystem has received all the data in a transaction. In Write-Back caching, the controller sends a data transfer completion signal to the host when the controller cache has received all the data in a transaction. The controller then writes the cached data to the storage device in the background.
    The risk of using Write-Back cache is that the cached data can be lost if there is a power failure before it is written to the storage device. This risk is mitigated by using a BBU on PERC H700 or H800 cards. Write-Back caching has a performance advantage over Write-Through caching. The default cache setting for virtual disks is Write-Back caching. Certain data patterns and configurations perform better with a Write-Through cache policy.
    Write-Back caching is used under all conditions in which the battery is present and in good condition.
    Write-Through caching is used under all conditions in which the battery is missing or in a low-charge state. Low-charge state is when the battery is not capable of maintaining data for at least 24 hours in the case of a power loss.
    Write-Back mode is available when the user selects Force WB with no battery. When Forced Write-Back mode is selected, the virtual disk is in Write-Back mode even if the battery is not present. It is recommended that you use power backup system when forcing Write-Back to ensure there is no loss of data if the system suddenly loses power.

    http://www.dell.com/downloads/global/products/pvaul/en/perc-technical-guidebook.pdf

     

     

     

     


    Regards, Dave Patrick ....
    Microsoft Certified Professional
    Microsoft MVP [Windows]
    Saturday, August 27, 2011 2:59 AM

All replies

  • DELL PERC H700 and H800 Technical Guide 20
    4.10 Virtual Disk Write Cache Policies
    The write cache policy of a virtual disk determines how the controller handles writes to that virtual disk. Write-Back and Write-Through are the two write cache policies and can be set on virtual disks individually.
    All RAID volumes will be presented as Write-Through (WT) to the operating system (Windows and Linux) independent of the actual write cache policy of the virtual disk. The PERC cards manage the data in cache independently of the operating system or any applications. You can use OpenManage or the BIOS configuration utility to view and manage virtual disk cache settings.
    In Write-Through caching, the controller sends a data-transfer completion signal to the host system when the disk subsystem has received all the data in a transaction. In Write-Back caching, the controller sends a data transfer completion signal to the host when the controller cache has received all the data in a transaction. The controller then writes the cached data to the storage device in the background.
    The risk of using Write-Back cache is that the cached data can be lost if there is a power failure before it is written to the storage device. This risk is mitigated by using a BBU on PERC H700 or H800 cards. Write-Back caching has a performance advantage over Write-Through caching. The default cache setting for virtual disks is Write-Back caching. Certain data patterns and configurations perform better with a Write-Through cache policy.
    Write-Back caching is used under all conditions in which the battery is present and in good condition.
    Write-Through caching is used under all conditions in which the battery is missing or in a low-charge state. Low-charge state is when the battery is not capable of maintaining data for at least 24 hours in the case of a power loss.
    Write-Back mode is available when the user selects Force WB with no battery. When Forced Write-Back mode is selected, the virtual disk is in Write-Back mode even if the battery is not present. It is recommended that you use power backup system when forcing Write-Back to ensure there is no loss of data if the system suddenly loses power.

    http://www.dell.com/downloads/global/products/pvaul/en/perc-technical-guidebook.pdf

     

     

     

     


    Regards, Dave Patrick ....
    Microsoft Certified Professional
    Microsoft MVP [Windows]
    Saturday, August 27, 2011 2:59 AM
  • Hi Dave,

     

    Thanks for the detailed info on the Perc behavior with Write Caching enabled.  That helps explain things a bit better for me. 

    I still have a question on what is affected when you have Write-Caching enabled on the Perc, but also have "Enable write caching on the device" check in Disk Management>disk properties>Policies Tab of Windows Server 2008.     The note above says "PERC cards manage the data in cache independetly of the operating system or any applications".  Does this mean it renders any OS write caching settings useless?  Or is it just saying that it does not have any interaction with any write-cache settings in the OS?

    Is there another level of write caching that goes on at the OS level when this is checked?  If Write-Back caching is enabled at the PERC level, and Write-caching policy checked at the OS level, is there a preformance hit because the process is repeated twice on every data transaction?  

    Monday, August 29, 2011 1:39 PM
  • As far as I know it is the controller driver that exposes this policy so should be one-and-the-same. May want to ask Dell support on this to confirm.

     

     

     

     


    Regards, Dave Patrick ....
    Microsoft Certified Professional
    Microsoft MVP [Windows]
    Monday, August 29, 2011 3:08 PM