Hola, yo soy Elver Sena Sosa and we going to talk about erasure coding.
Erasure coding, we usually know it as RAID 5 or RAID 6.
Ah when we are doing a RAID 5 protection that means we can tolerate one failure.
And when we are doing RAID 6 protection it means we can tolerate two failures.
Okay, now when you want to configure an SPBM so you can leverage erasure coding RAID 5 or 6.
If you select FTT of one, then you have a RAID 5 option for the method.
If you are selecting FTT of two you have a RAID 6 for the method.
So, let’s pretend we have our virtual machine half of VMDK object.
VMDK object.
We want to provide SPBM of one, FTT of one with a RAID of 5.
So, what vSAN will do is that it will take that object and it will break it up to three different components.
So, think of the hard drive, break it up and have one, two, three.
So, you have component one, component two, and component three.
In addition to that there will be a fourth component which is going to be the parity component.
So now vSAN will go and look at the vSAN cluster and say do I have three domains, three entities
that we can place these components to.
In this case, I’m sorry four entities.
We are looking for four hosts.
That means we can go here, here, here, here, here, or here.
Any of them will do.
So, let’s assume vSAN decides to put component one here, and component two over here, and
component three over here, and we need another parity we’ll put it, ah we’ll put it here.
Okay so now, when the virtual machine does a write into the hard drive, that will be
captured, it will go through the hypervisor process, go through ESXi, ESXi, ESXi, which
will then go to the vSAN process.
Which it’s going to ask itself the writes that are coming in are for blocks in component
one, component two, or component three.
It’s going to ask itself that question.
Once it decides which of those components get the write, the host will send the write
to the particular component.
Which then would also go send a write parity disc, so it sends two.
One to the component where the blocks are and one to the parity disc, so the parity
host can calculate the parity for that particular block.
If the write has blocks in different components, then the write will go those components.
And that is how RAID 5 works in the vSAN environment.
If now, and this is one of beauties about vSAN with its SPBM process, is that two weeks
later you realize that VM actually we did it wrong we don’t need an FTT of one, we
need an FTT of two with RAID 6.
All you need to do is go to that VM’s policy, change it from FTT of one to FTT of two.
Change the method of RAID 6, click okay, and automatically vSAN will go ahead and say well
now I need another component, another data component, component four.
And I need a second parity, parity two, vSAN decides we need that.
So vSAN is going to chop this up into four different pieces and go okay I need four host
to put the components in and I need 6 hosts for the parity.
So, I have component one, two, three.
I need component four which could go here or here.
Let’s say we put it here.
And I need parity two so we going to put it here.
And just like before when the write comes from VM, the processing host is going to determine
which component is written to and which parity it needs to send the information to.
Now one thing about this process is that because we went from a RAID 5 to a RAID 6 when you
redistribute the components that also means that vSAN is going to take the assisting components
and it’s going to move some blocks around to make sure these components get distributed
accordingly right, right.
So, before components when it was RAID 5, components one, two, three hold one hundred
percent of the object data.
Now components four, one, two, and three hold one hundred percent of the data.
So, these original components one, two, three, some of those get reassigned and repopulated
all across.
And with that that’s how erasure coding is done on vSAN.
Thank you for watching.
Soy Elver Sena Sosa.