This document is a DRAFT and does not reflect current School policy

Storage Costs Calculations

This document details the reasoning behind the charges made to research groups for file space on the Informatics storage servers.

Disk Storage

These charges are calculated on the basis the research groups requiring storage should pay a charge proportional to the cost of the storage they are using across the expected life of the hardware providing the storage, multiplied by the number of years they require the storage. In other words, should a group require the entire storage of a server for the whole of it's expected lifespan, then they should end up paying the entire cost of the server.

Looking at the hardware currently in use within the School or expected to be introduced within this financial year , these figures come out at:

Storage Type model Spec raw storage Available storage Cost (1) Cost per TB per Year (1)(2)
RW Pe730XD 12 x 4TB 48 22 (3) 8342 76
RW MD1400 12 x 6TB 72 33 (3) 8899 (4) 54
RW Combined PE730XD + MD1400 12 x 4TB + 12 x 6TB 120 55 (3) 17241 (4) 63
RW PE740XD2 24 x 8TB 192 88 (5) 19260 44
Mirror Pe730XD 12 x 4TB 48 32 (6) 8342 52
Mirror MD1400 12 x 6TB 72 48 (7) 8899 (4) 37
Mirror Combined PE730XD + MD1400 12 x 4TB + 12 x 6TB 120 80 (8) 17241 (4) 43

Storage Type Average Cost across Hardware Types per TB per Year (1)(9)
RW 60
Mirror 44

(1) Includes VAT

(2) Assumes hardware will be in service for 5 years

(3) RAID 10

(4) Includes cost of H730 HBA

(5) Estimate

(6) RAID 5

(7) RAID 6


(9) Rounded up

Tape Backup

This is considerably harder to calculate, not least because it is expected that the tape backup hardware will be refreshed at some point during 2020.The following base assumptions are made:

  1. We will be moving to LTO8 tape technology
  2. The retention period of backups are 6 months for incremental and 12 months for full backups
  3. The amount of data being backed up will not increase by more than 40% in the 5 year life of the new hardware

We have a quote for LTO8 tapes of 167.40 each (including VAT). Each LTO8 tape has a raw capacity of 12TB. Assuming that the life of the tape is 5 years, the tape will supply a total storage over the 5 years of 60TB The cost/TB/Year is therefore 167.40/60 = 2.79. Over the course of a year 12 full backups will be taken, each using 1TB so the total cost of full backups for a year is 12 x 2.79 = 33.48. We must also take into account that incremental backups also use tape. This will be difference for every dataset of course but an analysis of current usage suggests that on average, an incremental backup is 3.2% of the total data. Incremental backups are taken daily but recycled after 6 months so over the course of a year, incremental backups of 1TB will use (3.2 X 180) / 100 = 5.76TB.The cost of incremental backups over a year is therefore 5.76 x 2.79 = 16.07. The grand total is 33.48 + 16.07 = 49.55. Finally, there is the cost of the tape library to consider. As stated above, this is tricky to estimate as we are currently intending to purchase new hardware this year but the fairest way to calculate this would seem to be to divide the total cost of the library by the maximum possible amount backed up over the lifetime of the library. Teradactyl suggest that 190 tape will meet our needs for the next 5 years. This is a total amount of storage of 190x12x5 = 11400TB. We have a quote to replace the drives in out existing tape library with LTO8 drives, and provide maintenance for 5 years of 160066.24. The cost/TB is therefore 14.04. This gives a total cost of 63.59 for backing up to tape 1TB of storage for 1 year.

-- CraigStrachan - 30 Jan 2020

Topic revision: r6 - 13 Feb 2020 - 16:34:54 - CraigStrachan
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