5 Replies Latest reply on Apr 22, 2014 4:31 AM by depping

    VSAN Reads and Writes

    fubar2 Lurker

      I've read as much as I can find on Cormac and Duncan's blogs, but I still have a couple of questions about VSAN reads and writes:

       

      Is there any integrity checking for reads?  I haven't seen anything about error checking or other methods when data is read to make sure there hasn't been any corruption.  I know that increasing FTT results in more copies being stored, but my understanding is that a block being read will only happen from one copy, and only relies on the 2nd+ copy in the event of a disk/disk group failure.

       

      The information I've seen about writes is that 30% of the flash device is used for write buffer, and Cormac noted in his blog that the data is de-staged to HDD at regular intervals.  Is there any kind of sizing information about how many HDDs are needed?  For example, if I have 1000 desktops doing 10 write ops per second, do I need to have enough HDDs to satisfy 10000 write IOPS?  Or does the write buffer do some sort of random->sequential translation so that I need less write IOPS?

       

      Do all of the HDDs get used equally?  Is it possible for a HDD in a disk group to get full before the others?  I think I understand it correctly that unless striping is being used a VMDK will always write to the same HDD in the disk group, even if the disk group has multiple drives?  Or even without striping turned on will the writes for a VM use all of the HDDs in a disk group?

       

      Thank you for taking the time to answer my questions.

        • 1. Re: VSAN Reads and Writes
          crosdorff Enthusiast

          fubar2 wrote:

          Is there any integrity checking for reads?  I haven't seen anything about error checking or other methods when data is read to make sure there hasn't been any corruption.  I know that increasing FTT results in more copies being stored, but my understanding is that a block being read will only happen from one copy, and only relies on the 2nd+ copy in the event of a disk/disk group failure.


          I'm also interested in the data integrity question.

          With failures to tolerate and disk stripes per object set to 1, the vmdk resides on only 2 physical disks. Which mechanisms are in place to prevent (silent) data corruption?

          • 2. Re: VSAN Reads and Writes
            fubar2 Lurker

            Anyone?  I was hoping to at least find out about reads, it would be nice to have that reassurance.  The writes I can figure out through testing, even though I was hoping to have a better way than testing every workload to determine the right amount of HDD.

             

            Is there anyone I can reach out to?  Open a support case maybe?

            • 3. Re: VSAN Reads and Writes
              skx Novice

              I'm also wondering about this - i am hoping the book on VSAN will address this, this summer when it's released.

              • 4. Re: VSAN Reads and Writes
                depping Guru
                User ModeratorsVMware Employees

                fubar2 wrote:

                 

                Anyone?  I was hoping to at least find out about reads, it would be nice to have that reassurance.  The writes I can figure out through testing, even though I was hoping to have a better way than testing every workload to determine the right amount of HDD.

                 

                Is there anyone I can reach out to?  Open a support case maybe?

                 

                If you need these details then the best thing you can do is reach out to your VMware account team and ask them to get in touch with product management.

                • 5. Re: VSAN Reads and Writes
                  depping Guru
                  User ModeratorsVMware Employees

                  skx wrote:

                   

                  I'm also wondering about this - i am hoping the book on VSAN will address this, this summer when it's released.

                  Unfortunately certain aspects are under NDA and we can disclose these in public, or at least it is not up to us to do this.