Normally, from hardware RAID view point, we can make RAID 5 virtual disk using three drives.
but on vSAN, at least we need 4 nodes on it.
could you advise me?
As a matter of fact, the RAID5 configuration in vSAN does not use a Witness component as it is not required. But what you say about RAID5 being different when using object based storage over real storage is 100% correct.
In vSAN, we are implementing a form of RAID, but it cannot be called RAID really, as we are not working with disks (the D in RAID). We are working with objects and components of those objects. A more correct term would be RAIN (or Redundant Array of Independent Nodes). In the vSAN implementation of RAID5, we create 4 components of the parent vDisk object. The data is spread across these components in a manner similar to that of RAID5 in the disk world. To guarantee redundancy, each of the components must be placed on a different host, so a single host failure would take more than one component offline.
A witness is not required because RAID5 only allows for 1 failure. As there are 4 components, we would always need to have 3 out of the 4 components available. If we had a split-brain, then each partition would only have 2 out of 4 components, making the data useless in both partitions.
4 nodes, but all-flash nodes as well. vSAN is not RAID in that it is object-based storage, so a witness is still necessary. In RAID erasure coding systems where you're dealing with simply blocks, that's not necessary. There's a really good article on some of this written by Christos Karamanolis last February available here for more information.
Hi daphnissov,
Thanks give me message. of course, I already know basic knowledge of vSAN Erasure coding. ( AF only, need advanced and above license, need at least 4 nodes, 10Gbps network must)
My question is, why we can't use Erasure coding with 3 nodes vSAN cluster. because for example Dell PowerEdge RAID controller H730, H710, PERC 6, 5 and more....not only Dell product, but also other hardware vendor Hardware RAID controller, we can use RAID 5 using 3 drives, it is very well know rule for RAID 5.
You're trying to compare an object-based storage system with a block storage system. They aren't the same thing. vSAN erasure coding isn't really RAID, but it behaves in an equivalent manner. In a RAID subsystem implementing level 5 or 6, there is distributed parity just as there is in vSAN, but there is no witness component. The two implementations of technology providing somewhat equivalent outcomes is radically different.
As a matter of fact, the RAID5 configuration in vSAN does not use a Witness component as it is not required. But what you say about RAID5 being different when using object based storage over real storage is 100% correct.
In vSAN, we are implementing a form of RAID, but it cannot be called RAID really, as we are not working with disks (the D in RAID). We are working with objects and components of those objects. A more correct term would be RAIN (or Redundant Array of Independent Nodes). In the vSAN implementation of RAID5, we create 4 components of the parent vDisk object. The data is spread across these components in a manner similar to that of RAID5 in the disk world. To guarantee redundancy, each of the components must be placed on a different host, so a single host failure would take more than one component offline.
A witness is not required because RAID5 only allows for 1 failure. As there are 4 components, we would always need to have 3 out of the 4 components available. If we had a split-brain, then each partition would only have 2 out of 4 components, making the data useless in both partitions.