Push­ing past 3TB of disk space

Mac Format - - APPLE TALK -

I have a Mac Pro con­nected to a drive en­clo­sure con­tain­ing two 3TB disks. The disks were orig­i­nally set to a RAID 1 con­fig­u­ra­tion us­ing Disk Util­ity, so that each one backs up the other au­to­mat­i­cally. I am now fast ap­proach­ing the point where my stor­age needs ex­ceed the 3TB. The en­clo­sure has room for another two disks and I’m think­ing about mov­ing up to a RAID 5 con­fig­u­ra­tion. But Disk Util­ity doesn’t seem to sup­port this. Is there a third­party util­ity that will do this for me? Gareth Wood­ward Yes. SoftRAID (£126, softraid.com) will do this but I’m not sure it’s re­ally what you want. RAID 5 dis­trib­utes your data evenly across all the disks in the ar­ray, with some ex­tra par­ity data in­cluded so that the ar­ray can con­tinue to func­tion if any sin­gle drive fails. If you added another two 3TB drives, the par­ity in­for­ma­tion would take up the equiv­a­lent of one disk’s worth, leav­ing you with 9TB of stor­age – which seems like a good deal, but RAID 5 isn’t a backup. No flavour of RAID pro­tects your data. What they do is pro­tect ac­cess to your data. If a drive fails, you can still get at all your data with no in­ter­rup­tion. But un­til you have re­placed that drive and waited the day or two for the par­ity in­for­ma­tion to be re­built, your drives are ef­fec­tively in a RAID 0 ar­ray. Suf­fer a sec­ond drive fail­ure in this pe­riod and you’d lose all your data, be­cause ev­ery file is dis­trib­uted across the three re­main­ing disks. With one of those miss­ing, you have just two-thirds of ev­ery file. Which is the same as no file at all.

A safer op­tion is to set up two of the disks as a sin­gle 6TB vol­ume (a ‘con­cate­nated disk set’ in Disk Util­ity) to store files. Set up the re­main­ing disks in the same way, and use a util­ity such as Car­bon Copy Cloner (£27.50, bombich.com) to pe­ri­od­i­cally back up the first vol­ume to the sec­ond one. This gives you an ac­tual backup that will pro­tect your 6TB of data against the loss of up to two disks in the same set, though still not against one disk in each ar­ray.

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