vSphere Sizing Formula – Storage & Datastores

This blog post continues on from my previous blog post on vSphere Sizing Formula – CPU & RAM, this is again for me to share with you my methodology when it comes to sizing the requirements for a storage design.

I’m not going to go into specific storage protocols as this can be influenced by a multitude of design decisions.

Step 1 – Datastore Size

The answer to this isn’t a one size fits all, but let me give you my considerations.

Restore Time Objective

Your RTO objective should help determine define your VM size.  Let me explain what I mean, by walking over an example.

Your RTO time is an hour.  You have VM’s which are 2TB in size because they have multiple disk partitions in the guest operating system.  You currently backup using LTO-4 tapes which produce a restore rate of 120/MB’s.

Restore Rate x 60 Seconds x 60 minutes = Restore Amount

120 x 60 x 60 = 432,000 MB

Restore Amount = Resore Time Objective? Yes/No

432,000 MB (422GB) = No Restore Time Objective has been violated

Knowing this we would then dictate the VM would be changed to at least 5 x VM’s to meet the required RTO.

So this then determines our maximum VM size to be 422GB to meet RTO.

VM Size

Now we know our RTO and maximum VM size of 422GB, we then move onto the required space for our VM.

Maximum VM Size – Swap File = Actual Maximum VM Size

422GB – 8GB = 414GB

Buffer Space

Buffer space is the amount of space we need on the datastore for items such as:

• Log Files
• Snapshots
• Storage vMotion (temporary move space)

Rule of thumb on this is 25%.

Queue Depth

Jason Boche (@jasonboche) wrote an excellent article which can affect performance called  VAAI and the Unlimited VM’s per Datastore Urban Myth

Queue Depth for HBA’s is defaulted at 32, please check with your vendor if this should be altered for a vSphere environment.

Queue Depth for Software iSCSI Adapter is defaulted at 128, again check with you vendor is this should be altered in a vSphere.

Active IO Per VM x Number Of VM’s = Overall Active IO

9 x 500 = 4500

Overall Active IO / Number Of Hosts = Average Active IO Per Host

4500 / 12 = 375

Average Active IO Per Host / Queue Depth – Growth (Spike) % =  VM’s Per Datastore

375 / 32 – 50% = 6

Datastore Size

VM’s Per Datastore x Actual Maximum VM Size + Buffer Size = Datastore Size

6 x 414GB + 25% = 3TB

Step 2 – Performance Data Collection

I tend to look at performance first then capacity second, I have discounted capacity for this blog post as this should be straight forward to work out based around the RAID type you have chosen to use.

For the basis of the data collection we are going to make the following assumptions.

Disk	IOPS
7.2K SATA/NearLine SAS	75
10K SAS	125
15K SAS	150
SSD	2,500

Table To Show RAID Penalty

RAID Type	Write Penalty
0	1
1	2
5	4
6	6
10	2

Read/Write Collection

This is obtaining the key metrics from your physical systems, application owners, current storage array network, or perhaps its from perfmon or VMware Capacity Planner.

Read MB/Sec + Write MB/Sec = Overall MB/Sec

83 MB/Sec + 12 MB/Sec = 95 MB/sec

Read MB/Sec / Overall MB/Sec = Read Percentage

83 MB/Sec / 95 MB/Sec = 87%

Write MB/Sec / Overall MB/Sec = Write Percentage

12 MB/Sec / 95 MB/Sec = 13%

Front End IOPS Collection

Front End IOPS are the I/O transfers per second that your collection tool will see.  In this case we will use 1,231 IOPS.

Back End IOPS Collection

Back End IOPS is the performance that your SAN/NAS needs to accommodate taking into account RAID write penalty.

Read % + (RAID Penalty x Write %) = RAID Penalty Percentage

RAID 1: 87% + (2 x 13%) = 113%

RAID 5: 87% + (4 x 13%) = 139%

RAID 6: 87% + (6 x 13%) = 165%

RAID 10: 87% + (2 x 13%) = 113%

RAID Penalty Percentage x Front End IOPS = Back End IOPS

RAID 1: 113% x 1,231  = 1,391 IOPS

RAID 5: 139% x 1,231  = 1,711 IOPS

RAID 6: 165% x 1,231  = 2,031 IOPS

RAID 10: 113% x 1,231  = 1,391 IOPS

Step 3 – Target Disk Requirements

This is the process of determining which disk types meet your performance requirements.

Back End IOPS / Disk IOPS = Number Required Hard Drives

Table To Show Hard Drives Per RAID Type

RAID Type	Back End IOPS	Disk IOPS	Number Required Hard Drives
10	1,391	75 IOPS (7.2K SATA)	19
10	1,391	125 IOPS (10K SAS)	12
10	1,391	150 IOPS (15K SAS)	10
1	1,391	2,500 IOPS (SSD)	2 (to meet RAID 1 disk requirements)
5	1,711	75 IOPS (7.2K SATA)	23
5	1,711	125 IOPS (10K SAS)	14
5	1,711	150 IOPS (15K SAS)	12
5	1,711	2,500 IOPS (SSD)	3 (to meet RAID 5 disk requirements)
6	2,031	75 IOPS (7.2K SATA)	28
6	2,031	125 IOPS (10K SAS)	17
6	2,031	150 IOPS (15K SAS)	14
6	2,031	2,500 IOPS (SSD)	4 (to meet RAID 6 disk requirements)

Note: RAID 1 hasn’t been included apart from for SSD due to IOPS performance.

Step 4 – Growth Requirements

This is the amount of performance or capacity increase that is required to meet business or application objectives, in this case we will use 50%.

RAID Type	Back End IOPS	Growth %	Required IOPS	Disk IOPS	Number Required Hard Drives
10	1,391	50%	2,087	75 IOPS (7.2K SATA)	28
10	1,391	50%	2,087	125 IOPS (10K SAS)	17
10	1,391	50%	2,087	150 IOPS (15K SAS)	14
1	1,391	50%	2,087	2,500 IOPS (SSD)	2 (to meet RAID 1 disk requirements)
5	1,711	50%	2,567	75 IOPS (7.2K SATA)	35
5	1,711	50%	2,567	125 IOPS (10K SAS)	21
5	1,711	50%	2,567	150 IOPS (15K SAS)	18
5	1,711	50%	2,567	2,500 IOPS (SSD)	3 (to meet RAID 5 disk requirements)
6	2,031	50%	3,047	75 IOPS (7.2K SATA)	41
6	2,031	50%	3,047	125 IOPS (10K SAS)	25
6	2,031	50%	3,047	150 IOPS (15K SAS)	21
6	2,031	50%	3,047	2,500 IOPS (SSD)	4 (to meet RAID 6  disk requirements)

Considerations

Most storage vendors will have some kind of caching, you can use this to decrease the number of disks required or you can use it as an added performance bonus.

Latency hasn’t been taken into account, the rule for this is the less hops/distance travelled should equal less latency e.g. DAS is faster than SAN.