1. Bernd Panzer-Steindel, CERN/IT
Disk versus Tape
Status and Outlook
14.Nov.2005
Bernd Panzer-Steindel, CERN/IT

2. Bernd Panzer-Steindel, CERN/IT
Replace tapes with disks
CPU server
Disk server
storage cache
Tape server and drives
permanent tape storage
Replacement of the tape storage system, not the disk cache
Apply the same requirements :
large store, certain lifetime, limited performance,
high reliability and availability
14.Nov.2005
Bernd Panzer-Steindel, CERN/IT

3. Bernd Panzer-Steindel, CERN/IT
Disk storage models
NAS integrated disk server
~24 SATA disks
integrated into chassis
~3 RAID5 file systems
proven technology
today ~ 2.1 CHF/GB
‘extended’ CPU server
50 external disks, USB 2.0 or Firewire
connected via hubs
~7 RAID5 file systems
experimental system
(tested a 14 external disk system  works okay, but still lots of tests to be done…..)
today ~ 1.3 CHF/GB
14.Nov.2005
Bernd Panzer-Steindel, CERN/IT

4. Bernd Panzer-Steindel, CERN/IT
Disk storage cost evolution
cost increase
*2 for mirrored
*1.4 for RAID5
flat lines ??
14.Nov.2005
Bernd Panzer-Steindel, CERN/IT

5. Bernd Panzer-Steindel, CERN/IT
Input parameters for the cost simulation
evolution over 6 years, aggregate performance of 4 GBytes/s needed,
the growth rate is 5 PB in 2006 and than 10 PB per year
Disk
cost estimate for mid 2006 : 1.5 CHF/GB standard disk server, 1.0 CHF/GB
for special USB solution
lifetime of the disk server is 3 years, full replacement in the fourth year
price/performance improvement is a factor 1.5 per year
2. Tape
tape silos are ~ 0.5 MCHF for 6000 slots
tape drives plus servers are ~ 35 KCHF, average performance = 25 MB/s
cartridge size = 500 GB, CHF/GB
lifetime of drives is 5 years, replacement starts in the fifth year, all
previous data need to be copied to new cartridges
 1000 GB, 0.17 CHF/GB
14.Nov.2005
Bernd Panzer-Steindel, CERN/IT

6. Bernd Panzer-Steindel, CERN/IT
Cartridge lifetime and data copy
if one assumes a 4 year effective lifetime of tape drives and a linear
data growth, then the average lifetime of tape data is less than 3 years
( disk lifetime)
cost of the copy , example :
Assume : 10 PB of data, new cartridges cost 0.2 CHF/GB, copy needs to be done
within 300 days, drive performance is 25 MB/s (inefficiencies included)
Gain : double density tapes, 10 PB free space == 2 MCHF
Loss : PB in 300 days == 390 MB/s copy speed ==
16 input drives (model A) + 16 output drives (model B) ~ 1.3 MCHF
1 FTE + 48h disk buffer + extra tapes/slots ~ 0.3 MCHF
 Gain 2 MCHF Loss MCHF
Copy is mandatory at the end of a tape drive technology cycle.
In-between it might be cost effective to re-use the already existing tapes
at double density
There are some uncertainties in the calculations and it might not be cost-effective
14.Nov.2005
Bernd Panzer-Steindel, CERN/IT

7. Bernd Panzer-Steindel, CERN/IT
Accumulative cost models for different storage options
20% error bars
purchases are not
continuous, but discrete
points once or twice
a year
14.Nov.2005
Bernd Panzer-Steindel, CERN/IT

8. Bernd Panzer-Steindel, CERN/IT
Uncertainties
Disk price/performance evolution (assumed factor 1.5 ) :
flat curve ?, focus on 2 ½ and smaller disks, reliability, power consumption
large consumer market for external disks  audio, video, notebook trend
Tape price evolution
-- the difference between LTO-2 and LTO-3 is only 20% not a factor 2
-- re-use of the same cartridges with higher density possible but unclear
-- general future of tape storage in the market ( only high end)
……..
14.Nov.2005
Bernd Panzer-Steindel, CERN/IT

9. Bernd Panzer-Steindel, CERN/IT
Conclusions
The ‘USB-disk-model’ looks very promising, but needs more
investigation (also need much better power-consumption estimates).
Other models can be investigated
merge the CPU and disk servers
attachment of external storage to the CPU server
don’t use any RAID safety, rely on external copies of the data
The problem here is to understand the shift of costs
e.g. ‘cheaper’ hardware  software/operational complexity
The alternatives to tape storage are becoming attractive, but are still
immature.
Thus we will continue with tape storage for the next ~3 years and need to
test the alternatives thoroughly in parallel.
From the software and hardware point of view it will be transparent
and probably ‘not too complicated’ to have both solutions running in parallel.
14.Nov.2005
Bernd Panzer-Steindel, CERN/IT