The RAID 6 Areca ARC-1120 One-ups RAID 5 Controllers

Technical Specifications

Areca ARC-1120
Bus PCI-X 133 MHz
XOR Engine Intel 80332
Cache 128 MB DDR333, ECC
RAID Levels 0, 1, 0+1, 3, 5, 6, JBOD
Features Online RAID Roaming
Online RAID Level Migration
Online Stripe Size Migration
64 Bit LBA
Redundant Flash Image
Instant Availability / Background Init.
Browser-based RAID Management Softw.
Battery Backup Module optional
Notification: Email

The SATA II ports already offer locks.

The ARC-1120 In Practice


A RAID 6 rebuild process for two failed Western Digital WD740 Raptor-type hard drives took 25 minutes. Naturally, for the purpose of comparison, we also created a RAID 5 array, removed one drive and started the restoration process here too: at 21 minutes, the time needed was barely shorter. This puts the Areca controller on a par with the 9500-12S from 3Ware, which, given the same starting situation, also needed 21 minutes.

RAID Level Migration

RAID 6 requires two drives in the array to be used just for checksums, so it costs more storage than RAID 5. If space on the array runs out, a short-term solution is to convert the RAID 6 array to a RAID 5 one. This adds back the capacity of one drive, at the cost of reducing the fail-safe system from two drives to just one. To switch from RAID 6 to RAID 5, the ARC-1120 needed just under 54 minutes - not bad, considering the size of the RAID array (8 fast hard drives at 74 GB each).

If you start out by choosing the fast but zero-security RAID 0, you will be left out in the cold if you want to migrate. The reason is that introducing a RAID mode that works with parity data reduces the usable storage capacity of the array. If the RAID array were filled up with data, no more space would be available for the parity information. Switching from RAID 5 to RAID 6 is not easily done, for the same reason.

In contrast, it is easier to migrate from an existing RAID 5 array to RAID 3 or even to RAID 0, because the available capacity either stays the same or increases. If a migration frees up additional memory, it can be added to the RAID set after the migration process is finished.

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