Maximum Number of Disks In RAID 5

Posted on in Categories , , last updated January 21, 2011

Is there a limit to how many drives you can have in one RAID-5? What is the maximum number of physical drives recommended in a RAID-5 set?

Short answer – No, there is no limit.

Long answer – It all depends upon vendor implementation. Many high end servers and network storage devices can have hundreds of hard drives in them in RAID-5. 3Ware and other small server cards typically have the space of 7 drives. It depends on the controller. For example, some Proliant models can have a maximum 72 physical disk limit or other models of IBM server come with the limit of 96 physical hard disks. Recommended setup for RAID-5 to have seven drives with 1 drive as a hot spare.

Please note that the “parity” is very important here. It doesn’t matter how many drives there are, as long as only ONE fails, the data can be recovered provided that the rest stay up during your rebuild process. If two or more drives failed at the same time in RAID-5, you are doomed (hint: to avoid full disaster dump all YOUR DATA to a backup media).

Another practice is to use FC or external SAN controllers for large number of disks. If you are going to use 64 or more disk FC or SAN is standard practice. SCSI and FC (Fibre Channel) disks almost provide better performance, while SATA is good for storage intensive applications such as backup or large file servers.

So there is no single rule here as the recommended maximum number of disks in a RAID-5 system varies a lot and depends upon factors such as RAID Controller, RAID Controller CPU & memory, Disk technology and bandwidth.

4 comment

  1. That is good stuff. Did anyone ever check performance impact with mdadm. For example the diff. in having 5 disks in a raid5 compared to let’s say 25 disks in a raid5.

  2. Be cognizant of read errors as they are a serious issue with RAID. If the disk fails, the RAID will have to read all the disks that remain in the RAID group to rebuild. A read error during the rebuild will cause data loss since we need to read all other disks during rebuild.

    SATA disks experience read failure once every 12,5 TB of read operations, that means if the data on the surviving disks totals 12.5TB, the rebuild is certain to fail.

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