Performance - HP DL740 - ProLiant - 4 GB RAM Manual

hot plug RAID memory technology for fault tolerance and scalability

performance

ProLiant servers with Hot Plug RAID Memory technology use five memory controllers to
control five cartridges of industry-standard synchronous DRAM (SDRAM). When a
memory controller needs to write data to memory, it splits a cache line of data into four
blocks (shown as A, B, C, and D in figure 2). Then each block is written, or striped,
across four of the memory cartridges. RAID logic calculates parity information, which is
stored on the fifth cartridge. With the four data cartridges and the parity cartridge, the
data subsystem is redundant such that if the data from any DIMM is incorrect or if any
cartridge is removed, the data can be recreated from the remaining four cartridges.
figure 2: data striping in Hot Plug RAID Memory
Cartridge1
Cache Line
A1
B1
C1
D1
Hot Plug RAID Memory technology is implemented in ProLiant servers as part of a next-
generation chipset designed by HP that includes four application-specific integrated
circuits (ASICs). The ASICs enable the chipset to provide exceptional memory
performance, a high-level of fault tolerance, and hot-plug memory capabilities. Hot Plug
RAID Memory provides the ability for the memory subsystem to withstand a complete
memory device failure and to continue operating normally.
Although Hot Plug RAID memory is conceptually similar to RAID technology in disk drive
subsystems, there are some key performance and implementation differences between
Hot Plug RAID Memory and typical storage subsystem RAID.
Hot Plug RAID Memory does not have the mechanical delays of seek time and rotational
latency associated with hard disk drive arrays. Storage subsystem arrays use a single bus
to write the stripes sequentially across multiple drives. In contrast, Hot Plug RAID Memory
uses parallel, point-to-point connections to write data simultaneously across multiple
memory cartridges.
Also, Hot Plug RAID Memory eliminates the write bottleneck associated with typical
storage subsystem RAID implementations. In a storage array, the RAID controller
generally performs a read operation of existing parity before a write operation can be
completed. If a dedicated parity drive is being used, a bottleneck occurs. However,
because Hot Plug RAID Memory almost always operates on an entire cache line of data,
there is no need to read existing parity before a write operation. Therefore, no
performance bottleneck occurs.
When a traditional striped RAID storage subsystem rebuilds data, data is not protected
should another drive fail. However, Hot Plug RAID Memory operates in a typical
(nonredundant) ECC mode while data is being rebuilt. As a result, even if a secondary
memory failure occurs during a rebuild operation, the data is protected by ECC.
It is also important to note that like ECC memory protection, Hot Plug RAID Memory
protection creates only minimal performance overhead. In Hot Plug RAID Memory, a
RAID logic circuit calculates parity in parallel to the data flow, so error correction creates
almost no additional data latency.
Cartridge 2 Cartridge 3
A2 A3
B2 B3
C2 C3
D2 D3
Cartridge 4 Parity Cartridge
A4 A parity
B4 B parity
C4 C parity
D4 D parity
5