Hola, yo soy Elver Sena Sosa. Now we're going to talk about disk groups,
which is the unit of compute in a vSAN cluster. These groups are
actually a collection of disks that exist in the hosts in that cluster. So,
if you have an ESXi host, you will put a bunch of disks inside the host. But, the
disks themselves are put in this collection, this logical unit
of these groups, and in this group itself there is an architecture where
you have two levels. The first level is the cache level where you have an SSD.
There is just one disk, no more. And at this group there is only a single cache drive.
The SSD is your cache. In the second level, which
is the capacity tier you can have either SSDs or hard drives. Now, you can use
either one of them, but for performance purposes you
probably want to go with the SSDs. That will give you the best performance
you can get. This second tier, or the second level as you say, is called the
capacity. In the capacity tier you can have up to seven disks (a minimum of one and up
to seven.) And, a single ESXi host can have up to five of these groups in a single host.
Now if we were to do a quick map for vSAN, you can have seven of these on the disk
group in the capacity tier and then one cache. That means you can have eight
different discs, being that this single disk group times five. That is 40 disks
that you can have in a single hose providing storage to your vSAN and data
store, 35 of which will be for the capacity. Now you may ask yourself what's
the role of the cache and what's the role of the capacity disk? It all
comes down to performance. VMware decided to design vSAN
to be a very high-performance storage solution. VMware does not have
something like tier zero, tier one, tier two. vSAN does not have that. vSAN just
has fast and super fast. How do you differentiate between the fast and super
fast? Well, when the write comes in to a disk
group, it always goes into the cache. Always, 100% of the time they write is
going to come into the cache. The cache is going to turn around and acknowledge
that write. You get a write performance of the cache speed 100% of
the time. Now, the super fast difference comes in when
the read comes in. When a read comes in, and if the block that you are reading is not in
the cache, then the host that is hosting to this group is going to go out
to the capacity to find that block. Here is where the
difference between a hybrid solution, which is one where you have all hard
drives, or an all flash solution makes the difference. With a hybrid solution,
you can see that when you have to read from the capacity by being just regular
spindles, your read is going to be a bit slower than if you have an old
flash, which would get better performance. Now I know the question you might ask
yourself is, okay so the read comes into the cache, it is not there but it's in the
capacity. But, all the writes come in the cache.
So, when does the cache destage down to the capacity? Well, the cache is
going to start copying blocks from the cache into the capacity disk. When the
blocks are cold, that's one way that will start these destagings, which makes
sense. And the other one is when the cache starts running out of space.
When the cache starts running out of space or the capacity in the cache is
about 90-plus percent utilization, the cache would automatically start
destaging stuff into the blocks themselves, into the capacity tier of the
cells, to make sure that we have enough space here. And now one more thing I
didn't mention is that something that vSAN does so the reads can at
least be such fast as they can be, is that any time a read comes from the capacity tier
for the hybrid, the cache will copy those blocks and keep them in the cache so that
the next time they read comes in for that block, it will be in cache and the read
will be responded directly from the cache. So, you get better performance
that way. Something else about this is, well we have hybrid we have
all-flash, but what is different from the read? When the read comes in
from the all-flash, the blocks that were read don't get copied
back up to the cache. They stay in the capacity. Because of that, there is
a design decision that was made for the disk group on the cache.
Whereas, when you have a hybrid solution the cache by default is going to be
split 70/30. So, 70% of the space in the cache is reserved for the reading and
30% of the space in the cache is reserved for the write. Now, on the
all-flash, since there is no need to copy the blocks back to the cache, I mean this is
SSD anyway so they are reading fast regardless, there is no 70/30 split. It is 100%
write only at the cache here. That is kind of the difference from a
performance perspective between a hybrid solution and an all-flash. Now, a few things
to add, if you go hybrid or all-flash every thing in this group in the cluster
has to be the same configuration. It has to be either hybrid or all-flash, you cannot
mix and match those two. You can't do that. Also, the disk groups, the number of
capacity disks that you have in any disk group could
be different, it could vary. However, it is strongly recommended that these
groups in the cluster are homogeneous. So, they have more or less
the same number of capacity disks or more or less the same cache disks, so that
performance is consistent regardless of where you're storing the information. One
more thing is that when you have a hybrid you will have your ESXi hose
connect them to the network. Well, you will connect it to the network
regardless whether you have hybrid or flash. But, when you have a hybrid
one gigabit per second, it is supported for hybrid. When you have all-
flash, it has to be 10 gig per second. Of course if you go with the hybrid and
you do 10 gig per second you will have more capacity on your network,
but still your slowness is going to be when the capacity that reads come in.
That's kind of it for what is a disk group. I am Elver Sena Sosa, thank you
for watching.