Testing replacement of drives in hardware-based RAID systems


Hard disc drives are becoming more reliable, often achieving a Mean Time To Failure (MTTF) of up to 2.5 million hours and carrying a warranty of up to five years. In practice, this means that a storage system comprising 1000 identical hard drives should only expect two or three drives to fail during one year within the warranty period.

However, a few drives may still fail and once this occurs they must be replaced. If identical drives are available, either through inventory or a limited spares manufacturing program by the manufacturer then simple replacement is possible. Once it is no longer possible to source identical drives then drives with slightly different characteristics, such as later models must be used if possible. The challenge for IT managers is to understand which drives would work as suitable replacements in existing systems.

This challenge is more difficult for hardware controlled RAID systems. In order to promote a better understanding of the issue, Toshiba undertook some testing using some common RAID controllers including Broadcom’s LSI “MegaRAID SAS 9380” and the new Microsemi Adaptec “SmartRAID 3154”, which are considered functionally compatible with Adaptec’s “Series7” and “Series8” models.

A total of ten tests were performed using a RAID5 configuration consisting of four identical hard drives connected to either the Broadcom or Adaptec RAID controller. To simulate the scenario where a replacement was necessary, one hard drive was then replaced. It was then checked as to whether this drive had been accepted by the controller, whether a rebuild had been successfully completed and whether system performance was maintained.

At the end of the testing, Toshiba concluded that there is some flexibility when replacing failed disk drives in a RAID set. It is possible to replace 512n drives with 512e models as well as larger capacity or third-party models. Additionally, outdated SAS 6Gbit/s hard drives can be replaced with new models featuring a 12Gbit/s interface as the 12Gbit/s model is backwards compatible with the 6Gbit/s standard. However, it should be noted that 512n/512e models are not compatible with 4kn models. In addition, certain RAID controllers even allow replacement of SATA drives with SAS models and replacement of hard drives with SSDs. However, just because these configurations are supported by the RAID controller this does not mean they should be implemented in practice.

Toshiba has authored a report with full details of each test and its outcome.