Users Guide

● Cache Use
Hardware RAID controllers utilize cache (a temporary repository of information) for their normal operation. The normal
operation cache comprises DRAM memory, which, like system memory, retains data only when powered on.
Newer controllers utilize NVCache, which is utilized when the server is powered off. NVCache memory contains both DRAM
memory (for normal operation) and flash memory (non-volatile). The controllers battery (if operational) powers the DRAM
memory during a power loss so that the contents can be copied into the flash memory for indefinite storage.
The contents of cache can essentially be broken into three parts:
● RAID configuration and metadata - Information about the RAID arrays including configuration information, disk members, role
of disks, etc.
● Controller logs - RAID controllers maintain several log files. Dell technicians rely on the TTY log as the primary log for
troubleshooting various RAID and hard drive issues.
● RAID data - This is the actual data destined to be written to the individual hard drives. Data is written into the cache of the
controller in both Write Through and Write Back cache policy modes.
Slicing
Configuring multiple RAID arrays across the same set of disks is called Slicing.
RAID puncture
A RAID puncture is a feature of Dell PowerEdge RAID Controller (PERC) designed to allow the controller to restore the
redundancy of the array despite the loss of data caused by a double fault condition. Another name for a RAID puncture is rebuild
with errors. When the RAID controller detects a double fault and there is insufficient redundancy to recover the data in the
impacted stripe, the controller creates a puncture in that stripe and enables the rebuild to continue.
● Any condition that causes data to be inaccessible in the same stripe on more than one drive is a double fault.
● Double faults cause the loss of all data within the impacted stripe.
● All RAID punctures are double faults but all double faults are NOT RAID punctures.
Causes of RAID puncture
Without the RAID puncture feature, the array rebuild would fail, and leave the array in a degraded state. In some cases, the
failures may cause additional drives to fail, and cause the array to be in a non-functioning offline state. Puncturing an array has
no impact on the ability to boot to or access any data on the array.
RAID punctures can occur in one of two situations:
● Double Fault already exists (Data already lost).
Data error on an online drive is propagated (copied) to a rebuilding drive.
● Double Fault does not exist (Data is lost when second error occurs).
While in a degraded state, if a bad block occurs on an online drive, that LBA is RAID punctured.
This advantage of puncturing an array is keeping the system available in production till the redundancy of the array is
restored. The data in the affected stripe is lost whether the RAID puncture occurs or not. The primary disadvantage of this
88
Troubleshooting hardware issues