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

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