- Sun Storage Virtual Tape Library VTL User Guide

Managing virtual libraries

96267 • G Chapter 4 VTL operations 53

6. WhentheEnter virtual tape propertiesdialogappears,checktheTape

Capacity On Demandcheckbox(

E below).

Sunrecommendscapacityondemandformostusers. 

7. Ifyouarenotusingtapecaching,usethe Maximum Capacity spinnercontrol

(

F above)tosetthemaximumsi ze ofyourvirtualtapesto 50‐60GBortothe

averagesizeofyourbackupjobs(whicheverislarger). 

Thisapproachminimizes wastedspacein thecriticaldiskcache,wherethefirstcopy

ofeachbackupjobisstored.Itmaximizesthenumberof

cartridgesavailablefor

backupjobsatanygiventime.Yourcopy/vaultsoftwarecanthenconsolidatethese

smallervolumesontofull‐sizedphysicaltapecartridgesout sidethebackupwindow.

8. IfyouareusingVTLdatacompressionwithtapecaching,usethe Maximum

Capacity spinnercontrol(

F above)to reducetheMaximum Capacityto85‐90%

oftheencompassedcapacityoftheselectedmedia.

Intheexampleabove,youwouldreducemaximumcapacityto42 ‐43GBwhenusing

compression.

Whenyouenabletapecaching,asinglevirtualtapehastwoimages,onein thedisk 

cacheandone

inthetape cache.Bothimagesmustbelogically identical.Butwhen

youusecompressionwithtapecaching,youcannotbesurethatthediskandtape

imageswillbeidentical.Differentcompressionalgorithmscanproduceslightly 

differentcompressionratios.So,unlessyouallowa10‐15%marginofsafety,you



mightfindthatacompresseddatasetthatfillsatapeimageondiskwillnotfitona

physicalcartridge.

9. Ifyouareusingtapecaching,leavetheMaximum Capacity controlsettoth e

maximumcapacityofthephysicalmedia,unlessyouarealsousingVTLdata

compression.

Thediskandphysical

‐tapeimagesofavirtualtapemustbelogicallyidentica lwhen

youenabletapecaching.Theycannotbedifferentsizes.