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

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

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

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

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, 0.35 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

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 : 10 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 1.6 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

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

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

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