Differences between an Enterprise-Class HDD and a Desktop-Class HDD
Examples of Desktop-Class and Enterprise-Class HDDs
|Brand||Desktop-Class Variant||Enterprise-Class Variant|
|Hitachi Global Storage Technologies||HDS721010KLA330||HUA721010KLA330|
|Western Digital Corporation||WD10EACS||WD1000FYPS|
What is sector remapping?
HDDs today have a built-in ability of repairing defective sectors with a backup supply of sectors. This repair process is seamless to the user, and is not required for the user to perform sector remapping manually. However, given the two aforementioned environments, sector remapping times vary greatly between the two. With a Desktop-class drive, the HDD will attempt to retrieve the data and remap the sector which is damaged, requiring as much time as several minutes. During this sector remapping process, a HDD can time out and become unresponsive to the HDD controller. Because of this, a HDD can be ejected from the controller, as the Controller believes that the HDD is defective as it is unresponsive.
This effort of using several minutes in attempting to retrieve data from a defective sector is essential for single-drive usage, as there is not another copy of the data available. However this behavior is not beneficial in RAID environments, as a drive being ejected from the RAID Controller will result in the Volume becoming degraded, due to a few bad sectors.
To resolve this, sector remapping for Enterprise-class HDD is handled differently. On an Enterprise-class HDD, the HDD will attempt to retrieve the data from the affected sector, however if it cannot be retrieve under a few seconds, the HDD will mark the sector as defective and consider the data lost. The HDD maintains a lower time interval to perform sector remapping thus to avoid timing out or being ejected from the RAID Controller. This behavior is essential to maintain High Availability of the Volume, and to continue serving the data to customers and users.
After marking the sector defective, the HDD will then ask the controller to rebuild the missing data from the other Redundant drives (RAID 1, 5, 6 for example); the rebuilt data from the other drives will then be placed on a backup sector of the HDD.
How is sector remapping behavior handled with the Synology Disk Station Manager (DSM) ?
With the Synology DSM, error handling is either held by the disk itself, or the DSM can ask the hard drive to try again. When the DSM encounters a sector error on the hard disk, the DSM can wait for a few moments while the hard drive attempts to repair the defective sector. After the interval has expired, the DSM can either ask the HDD to try again to recover the sector, or eject the drive because it's not responding to the DSM, or too many defective sectors are encountered.
What happens to the data on the defective sector?
Whether using a Desktop-Class or Enterprise-Class HDD, if the sector is defective then the data which the the sector was holding is lost. While the Synology DiskStation will make multiple attempts to retrieve the data (by asking the HDD to retry the request), or the HDD itself will attempt multiple attempts to retrieve the data, the system (computer as a whole, or Synology system) cannot be held waiting forever in waiting for a sector to be repaired, data retrieved, or remapped. This will cause the Volume to degrade or cause system errors. If the sector has failed, then the data is lost. If a redundant volume is in use, the data from the defective sector can be rebuilt using the redundant mirrors.
What can be done to avoid data loss due to sector failure?
What are the other differences between Desktop and Enterprise Class HDDs?
The differences are of a varying degree, depending on the hard drive manufacturer - but in summary, most Enterprise-class HDDs have higher tolerance internal parts - designed to meet the needs of high-availability environment, 24/7. Also, higher tolerance for vibration, heat, and the other environmental factors which comes in a server environment. For additional details, please refer to the Intel paper at the end of this article.
Which class of hard drive should be used with the Synology DiskStation?
Either class of drive (Desktop or Enterprise) can be used within the Synology Disk Station; Desktop-class drives can be used for most customers and intentions of the Synology Disk Station. Enterprise-class variants should be for environments that require the high-resiliency internal parts of the drives as referenced by the Intel Paper, or the drive will be used where constant random access of data on the hard drive for every single second which the drive is active is expected.
To avoid sector remapping behavior on Desktop-Class HDDs, please consider reading the following article.