- Sun Storage Virtual Tape Library VTL User Guide

Advantages of VTL tape virtualization
96267 • G Chapter 1 Introduction: VTL appliances and enterprise data-protection 7
Operatingsystemsandbackupapplicationsthatcurrentlyinteractwithaphysical
libraryviaACSLScontinuetouseACSLSwhenaccessingVTLvirtuallibraries.
ACSLSfurt he rhidestheparticularsofdisk‐basedtapevirtualizationbehinda
standard,well‐knowninterface.
Thisabilitytomaskthelibrar yimpl emen tat ionfromclientapplicationsand
operating
systemsletsACSLSmakedisk‐basedtape‐virtualizationavailableto
platformsthatcouldnototherwise supportit.Platformsthatdonotsup portSCSI
passthroughlibrarycontrol,suchasUni sy s, Tangent,andmainframeoperating
systems,canbackuptoSunVTLvirtu a llibrariesusingACSLS.
Themorepowerful,more specializedli brary‐
controlfunctionalityof ACS LS 
improvestheavailabilityofsuitablyconfiguredVTLsolutions.ThebriefSCSIpath
interruptions that mayaccompanyfailoverandfailbackarelesslik el ytointerfere
withthebackupapplication,because ACS LS retriestheconnectiona gg res sively
enoughtorestorethepath.
Truetapevirtualizationwithdynamically
allocateddiskspace
Correctlyconfigured,dynamicallysizedvirtualtapevolumesprovidethehighest
capacityandperformance.WhentapesarecreatedwiththeVTLCapacityOn
Demandfeature enabled,theVTLsoftwareallocatesspaceasdataiswrittentodisk
ratherthanallatonce.Forinstance,aphysicaltapewithacapacityof400
‐GBcan be
emulatedwithoutallocatinganyspaceinitially,andthereafterenlargedas neededin
5‐GBinc reme nts (seethefigurebelow).
Thisapproachtospaceallocationhastwomajor ad vantages.First ,itminimizes
wasteddiskcapacity.Second,andperhapsmoreimportantly,itmaximizesarray
performanceandrelia bil it y.Dividingdata
intomult ip le segmentsdistributesitmore
evenlyacrossthearray,involvesmorevolumegroupsineachI/O,andreducesthe
averagelengthofeachseek duringI/O.
Space is not allocated on
disk until data is written
400-GB physical tape 01A00001
400-GB physical tape 01A00002
400-GB virtual tape 02B10001
= 5-GB header/data segment = 5-GB data segment
= 2-MB tape archive files
400-GB virtual tape 02B10000