Morning! welcome to virtualcloudblog.com and thanks for checking it out. Today, I’ll write this post about DELL ScaleIO, I’ll try to explain the difference between RWBD vs RWTD in ScaleIO Ready nodes.

What is RWBD?

RWBD  is write back cache

Write back caching: this caching technique improves the subsystem’s response time to write requests by allowing the controller to declare the write operation ‘complete’ as soon as the data reaches its cache memory. The controller performs the slower operation of writing the data to the disk drives at a later time.

From ScaleIO perspective, RWBD is the preferred opcion

What is RWTD?

RWTD is write through cache

Write trough Caching: when the controller receives a write request from the host, it stores the data in its cache module, writes the data to the disk drives, then notifies the host when the write operation is complete. This process is calles write-trough caching because the data actually passes through-and is stored in- the cache memory on its way to the disk drives.

 

How to check which option we have in our DELL ScaleIO servers?

  1. Putty the ESX host running a SDC agent.

  2. From the output: take a look the Virtual Drives and the Cache column . In this case host is configured to use RWBD.

How to change RWTD to RWBD?

  1. Putty the ESX host running a SDC agent in Maintenance Mode
  2. From the output: take take a look the Virtual Drives and the Cache column . In this case host is configured to use RWTD, so we need to change it.
  3. From ScaleIO, Enter the host in maintenance mode.
  4. From the SSH, run the following command.
  5. Output from the command is something similar as:
  6.  Once DONE, exit host from maintenace mode from ScaleIO and let the rebuild, rebalance complete to get the node back in production.
  7. Finish!

Please note the disk cache policy can be also changed/checked from iDRAC.

  1. Log in into the the iDRAC
  2. Go to: Configuration, Storage Configuration, Select the controller, Virtual Disk, select one virtual disk, and Edit Cache Policy.

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