IBM System Storage DS3500 Introduction And Implementation Manual page 84

7914DS3KPlanning_090710.fm
Attention: SAS nearline drives cannot be used as hot spares for the SAS 10KRPM or SAS
15KRPM drives. When intermixing SAS and SAS nearline you will want to create hot
spares for both types of drives.
Best practice: When assigning disks as hot spares, make sure that they have enough
storage capacity. If the failed disk drive is larger than the hot spare, reconstruction is not
possible. A best practice is to use the largest drive size for the drive type installed as hot
spare drives. See 10.3, "Set hot spare drive" on page 268 for more details on defining hot
spare drives.
Figure 3-19 Hot spare coverage with alternating loops
Planning for hot spares
Even though disk drives are becoming increasingly more reliable, disk failures can occur. To
protect your data in case of a drive failure, you should primarily use a redundant RAID level,
for example, RAID 1, 5, or 10. This way, your data will remain intact should a hard drive fail.
However, the array will then operate in a degraded state until the failed disk drive is replaced
and data reconstruction completes on the new disk drive.
To ensure as high availability as possible, we strongly recommend that you use hot spare
drives. Under normal circumstances, the hot spare drives do not participate in any array.
When one of the disk drives fails, the host spare will automatically be used as a replacement
drive. Data reconstruction onto the hot spare will take place immediately. When
reconstruction finishes, the array will again be in a fully operational status and the hot spare
will become a member of the array.
Depending on your configuration, it might be wise to use more than just one hot spare. In
small DS3500 configurations with a relatively low number of disk drives, one hot spare might
suffice. But for larger configurations, we recommend that you define several hot spares.
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IBM System Storage DS3500: Introduction and Implementation Guide
Draft Document for Review March 28, 2011 12:24 pm