On Fri, 05 Jul 2013, Benny Pedersen wrote to mark lewis:   
      
    ml> RAID is -=NOT=- a backup strategy...   
      
    BP> lol :)   
      
   it is not a laughing matter... not in the least...   
      
    ml> RAID is only for storage, access speed, and redundancy...   
      
   clarification : fault tolerance and performance   
      
    BP> i dont agree, it was a backup for me, i still in progress to   
    BP> migrade to another nas for now, 3.6TB to be moved just not take   
    BP> under one sec :)    
      
   [quote]   
   A RAID system used as secondary [sic] storage is not an alternative to backing   
   up data. In RAID levels > 0, a RAID protects from catastrophic data loss   
   caused by physical damage or errors on a single drive within the array (or two   
   drives in, say, RAID 6). However, a true backup system has other important   
   features such as the ability to restore an earlier version of data, which is   
   needed both to protect against software errors that write unwanted data to   
   secondary storage, and also to recover from user error and malicious data   
   deletion. A RAID can be overwhelmed by catastrophic failure that exceeds its   
   recovery capacity and, of course, the entire array is at risk of physical   
   damage by fire, natural disaster, and human forces, while backups can be   
   stored off-site. A RAID is also vulnerable to controller failure because it is   
   not always possible to migrate a RAID to a new, different controller without   
   data loss.[17]   
   [/quote]   
      
    ml> even data stored on a RAID must be backed up...   
      
    BP> in raid6 there is built in backup   
      
   incorrect... A RAID 5 uses block-level striping with parity data distributed   
   across all member disks. RAID 6 extends RAID 5 by adding an additional parity   
   block; thus it uses block-level striping with two parity blocks distributed   
   across all member disks. RAID 6 does not have a performance penalty for read   
   operations, but it does have a performance penalty on write operations because   
   of the overhead associated with parity calculations. Performance varies   
   greatly depending on how RAID 6 is implemented in the manufacturer's storage   
   architecture - in software, firmware or by using firmware and specialized   
   ASICs for intensive parity calculations. It can be as fast as a RAID-5 system   
   with one fewer drive (same number of data drives).   
      
   there is NO BACKUP in RAID... only Fault Tolerance (multiple copies of the   
   same data spread over multiple disks) and Performance Enhancement (access of   
   the same data over more than one platter at the same time). while you may   
   think that having multiple copies of the same data spread across multiple   
   drives is a backup, it is not... RAID is still suseptible to catastrophic   
   loss... it is possible to have very high fault tolerance but this still does   
   not negate catastrophic loss probabilities...   
      
   and software RAID? thanks but no thanks! the performance penalties are too   
   great for my liking... give me dedicated hardware RAID any day... then i can   
   RAID multiple RAIDs and have even more fault tolerance and performance... how   
   about a mirrored RAID5 of multiple RAID5s ;)   
      
   RAID5 - minumum 3 drives   
   Mirrored RAID5 (aka RAID5+1) - two RAID5s in mirror = 6 drives   
   RAID5 of RAID5s - min of 3 RAID5s each w/min of 3 drives = 9 drives   
   Mirrored RAID5 of RAID5s = 18 drives   
      
   and still none of the above provide backup functions... a true backup system   
   has other important features such as the ability to restore an earlier version   
   of data, which is needed both to protect against software errors that write   
   unwanted data to secondary storage, and also to recover from user error and   
   malicious data deletion. you just cannot get that from a RAID in any shape   
   form or fashion... why? because a malicious deletion or overwrite, for   
   example, is written to all drives in the array at the same time thus the data   
   is lost completely... in the case of deletion, one may be fast enough to   
   perform an OS level undelete operation on that file /if/ such operation is   
   even allowed at all... in the case of overwrite, good luck...   
      
   EOT   
      
   )\/(ark   
      
   --- FMail/Win32 1.60   
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