Raid faq

9
FAQ | RAID
What is the dierence between 6 Gb/s SAS controllers and 12 Gb/s SAS controllers?
Thedierenceisthat12Gb/scontrollershavetheabilitytotransfermoredata,orgreaterbandwidth.Themaximum
bandwidthofa6Gb/sportis600MB/s,whereasthemaximumbandwidthofa12Gb/sportis1200MB/s.Astandard
workstationwith3-4SASHDDsconnectedtothecontrollerwillseeverylittleperformancedierencebetween6Gb/sand
12Gb/scontrollers.Thelimitingfactoristherateatwhichtheharddrivescanreadandwritedata.Atthistime,harddrives
donotsaturatea6Gb/sbus,thereforelimitingthevalueofa12Gb/scontroller.Themostprobableusecasefora12Gb/s
controllerisinconjunctionwithanexternaldriveenclosurethatcontainsenoughdiskstosaturatethe12Gb/slinksofthe
controller.Ifthiscongurationisdesired,a12Gb/sSAScardwithexternalconnectionsshouldbeconsidered.
How does performance scale with drives and RAID level?
• RAID0–Performancescaleswiththenumberofdrivesused.e.g.for2drives,theperformanceisroughly2xthatofthe
onedrive.
• RAID1–Performanceisequaltotheslowestdriveinthemirror.
• RAID5–Performancescaleswiththenumberofdrivesused,minus1.
• RAID10–Performancescaleswiththenumberofdrivesused,dividedby2.
• RAID1E–Performancescaleswiththenumberofdrivesused,minus1.
What is the ideal combination of drives used in RAID volumes?
Ideally,alldrivesincludedinaRAIDarrayshouldbethesamein:
• Capacity
• Technology(SAS/SATA/PCIe)
• Drivevendor
How long does it take to initialize an array with high capacity disks?
Initializationtimes/ratesareasfollows:
IntelRAID1 160MB/s (1TBofRAID1HDDstakesabout1.6hours)
LSIRAID1 30MB/s (1TBofRAID1HDDstakesabout8hours)
Whilethesystemcanbeusedduringinitialization,doingsomayincreaseinitializationtime.Thesystemshouldnotbe
powereddownduringinitialization.Poweringdownduringinitializationmaycauseinitializationtorestartandprevent
completion of initialization on large arrays.
Why is the LSI RAID 1 initialization slower than the Intel RAID 1 initialization?
InitializationofRAIDintheLSIcontrollerishandledbythecontrollerinbackgroundmodeataslowratetominimize
impact on system operation.