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2 Bronze

## Fast Cache

andre.rossouw

I appreciate if you someone help with the these asked questions:

1- A customer wants to configure FAST Cache on their VNX storage system. Their working data set is 800 GiB in size. They plan to have sufficient FAST Cache to hold the working data set. There are currently no Flash drives in the storage system. The customer wants to follow EMC best practices.How can this be achieved?

Eight 200 GB Flash drives

Ten 200 GB Flash drives

Seventeen 100 GB Flash drives

Nineteen 100 GB Flash drives

2- A customer wants to add Flash drives to their VNX array and use them as FAST Cache. They want at least 850 GiB of FAST Cache capacity on each SP.There are no Flash drives in the array and they want to follow EMC best practices on hot spares.How can this be achieved?

Nine 200 GB Flash drives

Eleven 200 GB Flash drives

Seventeen 100 GB Flash drives

Nineteen 100 GB Flash drives

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I tried to find out the relation between skew and FAST cache but I could not find any useful information that gives me the correct answer among the provided answers...

I'm not sure if this is correct"FAST cache should be 5% of the active data " ..

I appreciate if you can explain the skew and FAST cache calculation .

1 Solution

Accepted Solutions
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3 Silver

## Re: Fast Cache

When dealing with FAST Cache questions, remember that it's configured as RAID 1, and that the capacity is evenly divided between SAP and SPB. Also bear in mind that a 100 GB drive doesn't have a data capacity of 100 GB - it's more like 91 GB.

So, if I have 17 Flash drives of 100 GB each, and I need a hot spare, that leaves 16 drives. They're RAID 1, so have the equivalent capacity of 8 drives - about 720 GB. Nineteen drives would have about 811 GB capacity.

Skew measures the distribution of I/O over the data area. As an example, if you told that there's 100 GB of data, and that the skew is 93%, you know that 93% of the I/O happens on 7% of the data. That 7% represents the working data set, in in this [trivial] example would be 7 GB. That's the data you'd like to see on Flash disks, so in a FAST Cache environment, that's how much FAST Cache you'd want.

Regards,

3 Replies
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3 Silver

## Re: Fast Cache

When dealing with FAST Cache questions, remember that it's configured as RAID 1, and that the capacity is evenly divided between SAP and SPB. Also bear in mind that a 100 GB drive doesn't have a data capacity of 100 GB - it's more like 91 GB.

So, if I have 17 Flash drives of 100 GB each, and I need a hot spare, that leaves 16 drives. They're RAID 1, so have the equivalent capacity of 8 drives - about 720 GB. Nineteen drives would have about 811 GB capacity.

Skew measures the distribution of I/O over the data area. As an example, if you told that there's 100 GB of data, and that the skew is 93%, you know that 93% of the I/O happens on 7% of the data. That 7% represents the working data set, in in this [trivial] example would be 7 GB. That's the data you'd like to see on Flash disks, so in a FAST Cache environment, that's how much FAST Cache you'd want.

Regards,

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6 Indium

## Re: Fast Cache

 Also bear in mind that a 100 GB drive doesn't have a data capacity of 100 GB - it's more like 91 GB.

Well, not really. A 100 GB drive actually has indeed 100 GB of space. Golnaz wrote it correctly by saying they have a working set of 800 GiB (remember the extra "i" ?). But I assume you meant to say that a 100 GB SSD actually has 91 GiB (with the extra i).

100,000,000,000 Bytes = 100,000,000,000 / (1024 x 1024 x 1024) = 93.13 GiB if such a drive were to be formatted with 512 Bytes per sector, whci they are not. In fact with Clariion and VNX that's 520 Bytes per sector, so the actual outcome is indeed 93.13 x 512 / 520 = 91.70 GiB net capacity for FAST Cache.

Mark the difference between GB and GiB !!! in VNX that means 100 GB disk = 91 GiB capacity

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2 Bronze

## Re: Fast Cache

I put the related question and answer also here:

1- A customer wants to configure FAST Cache
on their VNX storage system. Their working data set is 800 GiB in size. They
plan to have sufficient FAST Cache to hold the working data set.
There are currently no Flash drives in the storage system. The customer
wants to follow EMC best practices.
How
can this be achieved?

Eight 200 GB Flash drives

Ten 200 GB Flash drives

Seventeen 100 GB Flash drives

Nineteen 100 GB Flash drives

2- A customer wants to add Flash drives to their VNX
array and use them as FAST Cache. They want at least 850 GiB of FAST Cache
capacity on each SP.
There are no Flash
drives in the array and they want to follow EMC best practices on hot spares.
How can this be achieved?

Nine 200 GB Flash drives

Eleven 200 GB Flash drives

Seventeen 100 GB Flash drives

Nineteen 100 GB Flash drives

For Question 1, note that the working data
set is 800 GB, and that the working data has to fit into FAST Cache. Also
remember that FAST Cache is configured as RAID 1 sets, so we need double the
number of disks that will hold the data. In addition, we require a spare disk.
So, given that, remember that a 100 GB Flash drive holds around 93 GB, and a
200 GB Flash drive holds about 186 GB. To hold 800 GB, we require 9 100 GB
drives or 5 200 GB drives. The answer, then, would be 9+9+1 100 GB drives or
5+5+1 200 GB drives. The only answer that fits is the last one, nineteen 100 GB
drives.

For Question 2, note that the FAST Cache
requirement is 850 GB. Also remember that FAST Cache is configured as RAID 1
sets, so we need double the number of disks that will hold the data. In
addition, we require a spare disk. So, given that, remember that a 100 GB Flash
drive holds around 93 GB, and a 200 GB Flash drive holds about 186 GB. To hold
850 GB, we require 10 100 GB drives or 5 200 GB drives. The answer, then, would
be 10+10+1 100 GB drives or 5+5+1 200 GB drives. The only answer that fits is
the second one, eleven 200 GB drives.