On Fri, 05 Jul 2013, Benny Pedersen wrote to mark lewis:
RAID is -=NOT=- a backup strategy...
lol :)
it is not a laughing matter... not in the least...
RAID is only for storage, access speed, and redundancy...
clarification : fault tolerance and performance
i dont agree, it was a backup for me, i still in progress to
migrade to another nas for now, 3.6TB to be moved just not take
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]
even data stored on a RAID must be backed up...
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
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