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.
 
