|C H A P T E R 2|
Overview of Integrated Mirroring and Integrated Mirroring Enhanced
This chapter provides an overview of the LSI Integrated Mirroring (IM) and Integrated Mirroring Enhanced (IME) features. It includes these sections:
The LSI Integrated Mirroring (IM) and Integrated Mirroring Enhanced (IME) features provide data protection for the system boot volume to safeguard critical information such as the OS on servers and high-performance workstations. The IM and IME features provide a robust, high-performance, fault-tolerant solution to data storage needs.
The IM and IME features support one or two mirrored volumes per LSI SAS controller, to provide fault-tolerant protection for critical data. The two volumes can have up to twelve disk drives total, plus one or two hot-spare disks.
If a disk in an Integrated Mirroring volume fails, the hot swap capability allows you to restore the volume by simply swapping disks. The firmware then automatically re-mirrors the swapped disk. Additionally, each SAS controller can have one or two global hot-spare disks available to automatically replace a failed disk in the IM or IME storage volumes on the controller. Hot-spares make the IM/IME volume even more fault-tolerant.
|Note - You can also configure one IM or IME volume and one Integrated Striping (IS) volume on the same LSI SAS controller.|
The IM/IME feature operates independently from the OS, in order to conserve system resources. The BIOS-based configuration utility makes it easy to configure IM and IME volumes.
IM and IME support the following features:
1. Configurations of one or two IM or IME volumes on the same LSI SAS controller. IM volumes have two mirrored disks; IME volumes have three to ten mirrored disks. Two volumes can have up to 12 disks total. (Requires Integrated RAID firmware v1.20.00 or above.)
2. One or two global hot-spare disks per controller, to automatically replace failed disks in IM/IME volumes. (Support for two hot-spares requires Integrated RAID firmware v1.20.00 or above.) The hot-spares are in addition to the 12-disk maximum for two volumes per SAS controller.
3. Mirrored volumes run in optimal mode or in degraded mode (if one mirrored disk fails).
4. Hot swap capability
5. Presents a single virtual drive to the OS for each IM/IME volume.
6. Supports both SAS and SATA disks. The two types of disks cannot be combined in the same volume. However, an LSI SAS controller can support one volume with SATA disks and a second volume with SAS disks.
7. Fusion-MPT architecture
8. Easy-to-use BIOS-based configuration utility
9. Error notification: the drivers update an OS-specific event log.
10. SES status LED support
11. Write journaling, which allows automatic synchronization of potentially inconsistent data after unexpected power-down situations.
12. Metadata used to store volume configuration on mirrored disks.
13. Automatic background re-synchronization while host I/O continue.
14. Background media verification ensures that data on IM/IME volumes is always accessible.
The LSI Integrated RAID solution supports one or two IM/IME volumes on each LSI SAS controller (or one IM/IME volume and one Integrated Striping volume). Typically, one of these volumes is the primary or boot volume, as shown in FIGURE 2-1. Boot support is available through the firmware of the LSI SAS controller that supports the standard Fusion-MPT interface. The runtime mirroring of the boot disk is transparent to the BIOS, drivers, and OS. Host-based status software monitors the state of the mirrored disks and reports any error conditions. FIGURE 2-1 shows an IM implementation with a second disk as a mirror of the first (primary) disk.
FIGURE 2-1 Typical Integrated Mirroring Implementation
The advantage of an IM/IME volume is that there is always a second, mirrored copy of the data. The disadvantage is that writes take longer because data must be written twice. On the other hand, performance is actually improved during reads.
FIGURE 2-2 shows the logical view and physical view of an IM volume.
FIGURE 2-2 Integrated Mirroring Volume
An IME volume can be configured with up to ten mirrored disks. (One or two global hot-spares can be added also.) FIGURE 2-3 shows the logical view and physical view of an Integrated Mirroring Enhanced (IME) volume with three mirrored disks. Each mirrored stripe is written to a disk and mirrored to an adjacent disk. This type of configuration is also called RAID 1E.
FIGURE 2-3 Integrated Mirroring Enhanced with Three Disks
The LSI BIOS-based configuration utility enables you to create IM and IME volumes during initial setup and to reconfigure them in response to hardware failures or changes in the environment.
This section describes features of the LSI Integrated RAID firmware.
The Integrated RAID firmware allows host I/O to continue on an IM or IME volume while the volume is being re-synchronized in the background. Re-synchronization is attempted after a hot-spare is activated due to a physical device failure, or after a hot swap has occurred to a physical disk in the volume.
The firmware supports metadata, which describes the IM/IME logical drive configuration stored on each member disk. When the firmware is initialized, each member disk is queried to read the stored metadata in order to verify the configuration. The usable disk space for each member disk is adjusted down when the configuration is created, in order to leave room for this data.
The firmware supports hot swapping. The hot-swapped disk is automatically re-synchronized in the background, without any host or user intervention. The firmware detects hot swap removal and disk insertion.
Following a hot swap event, the firmware readies the new physical disk by spinning it up and verifying that it has enough capacity for the mirrored volume. The firmware re-synchronizes all hot-swapped disks that have been removed, even if the same disk is re-inserted. In a two-disk mirrored volume, the firmware marks the hot-swapped disk as the secondary disk and marks the other mirrored disk as the primary disk. The firmware re-synchronizes all data from the primary disk onto the new secondary disk.
SMART is a technology that monitors hard disk drives for signs of future disk failure and generates an alert if such signs are detected. The firmware polls each physical disk in the volume at regular intervals. If the firmware detects a SMART ASC/ASCQ code on a physical disk in the IM/IME volume, it processes the SMART data and stores it in nonvolatile memory. The IM/IME volume does not support SMART directly, since it is just a logical representation of the physical disks in the volume.
One or two disk drives per controller can be configured as global hot-spare disks, to protect data on the IM/IME volumes configured on the controller. If the firmware fails one of the mirrored disks, it automatically replaces the failed disk with a hot-spare disk and then re-synchronizes the mirrored data. The firmware is automatically notified when the failed disk has been replaced, and it then designates the failed disk as the new hot-spare.
The firmware supports a background media verification feature that runs at regular intervals when the IM/IME volume is in optimal state. If the verification command fails for any reason, the other disk’s data for this segment is read and written to the failing disk in an attempt to refresh the data. The current Media Verification Logical Block Address is written to nonvolatile memory occasionally to allow media verification to continue approximately where it left off prior to a power-cycle.
The firmware disables disk write caching by default for IM/IME volumes. This is done to increase data integrity, so that the disk write log stored in NVSRAM is always valid. If disk write caching were enabled (not recommended), the disk write log could be invalid.
The Integrated RAID firmware requires at least a 32K NVSRAM in order to perform write journaling. Write journaling is used to verify that the disks in the IM/IME volume are synchronized with each other.
The BIOS uses the LSI Fusion-MPT interface to communicate to the SAS controller and firmware to enable IM and IME volumes. This includes reading the Fusion-MPT configuration to access the parameters that are used to define behavior between the SAS controller and the devices connected to it. The Fusion-MPT drivers for all supported operating systems implement the Fusion-MPT interface to communicate with the controller and firmware.