Developers Guide

13 Understanding RAID with Dell SC Series Storage | 3104-CD-DS
3 Spare disks
Depending on the RAID level and the total number of disks in each SC Series storage array, one or more
spare disks are automatically configured and used in the event of a disk failure. The use of spare disks is
highly recommended as an additional level of protection should a disk failure happen. Spare disks will replace
the failed disk and allow the RAID set to rebuild. It is important to understand that two spare disks cannot
guarantee the survival of a RAID 10 or RAID 5 set that has a multi-disk failure event or a disk failure during a
RAID rebuild operation.
SC Series arrays automatically assign a single disk as a hot spare using these conventions:
 2U enclosures: One spare disk for every disk class (SSD, 15K, 10K, 7.2K, and so on)
 5U enclosures: One spare disk for every 21 disks. However, no single row contains more than one
spare disk.
3.1 No spare available
When a spare disk is not available, the following RAID conventions are used.
 RAID 6 is guaranteed to survive the simultaneous failure of any two disks. Data continues to be
available, but the set is degraded. A third disk failure in a degraded set can result in data loss.
 RAID 10 and RAID 5 are guaranteed to survive one disk failure per RAID set. Data continues to be
available, but the set is degraded. A second disk failure in a degraded set can result in data loss.
3.2 Disk failure
When a disk in a RAID set fails, the SC Series array takes the following actions.
If a spare disk is available: The spare automatically replaces the failed disk. Data from the failed disk is
reconstructed on the spare disk and continues to be available. During reconstruction, the set that contains the
failed disk is temporarily degraded. After reconstruction, performance returns to normal.
If a spare disk is not available: Data continues to be available, but the set is degraded.
If another disk fails in a degraded RAID 10 or RAID 5 set: With more disks present in the SC Series
system, chances of surviving a dual disk failure increase. However, there is no guarantee that the data will
stay online. If a second disk fails and that disk is required to rebuild the initial failing disk, then all I/O is halted
because the data is inaccessible. Contact Dell Support to attempt to data recovery in these situations.
If another disk fails in a degraded RAID 6 or RAID 10-DM set: The SC Series array continues to be
degraded until the failed disks are replaced. Data is reconstructed on the replacement disks. If the first
reconstruction is still underway, performance can be further reduced. After both disks are reconstructed,
performance returns to normal.
3.3 Replaced disk
When a failed disk is replaced, the SC Series array responds as described below.
If a spare disk was used: Data has already been reconstructed on the spare disk, so the new disk becomes
a spare.