HP DL740 hot plug RAID memory technology for fault tolerance and scalability - Page 5
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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 Cache Line Cartridge1 A1 B1 C1 D1 Cartridge 2 A2 B2 C2 D2 Cartridge 3 A3 B3 C3 D3 Cartridge 4 Parity Cartridge A4 A parity B4 B parity C4 C parity D4 D parity Hot Plug RAID Memory technology is implemented in ProLiant servers as part of a nextgeneration 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. 5
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