Flash Memories: NOR and NAND

Flash Memory

• NOR flash memories are a direct competitor of traditional EEPROM, randomly addressable, and typically used in BIOS.
• NAND flash memories offer higher storage density, but can only be read in blocks, making them cheaper per gigabyte and more common than NOR flash.
• NAND flash has a wear-out limitation of around 100,000 to 1 million recordings, but this can be expanded through uniform distribution of writes across blocks.

Solid-State Disks (SSDs)

• SSDs have replaced floppy disks and hard drives in mobile systems.
• They have become a popular choice due to their high I/O performance and low cost.

Clusters (I/O Servers) Evaluation

• The evaluation is based on a VME T-80 rack, used in the Internet Archive project.
• The storage node PetaBox GB2000 has 4× 500 GiB PATA disk drives, 512 MiB DDR266 DRAM, and a 1 GHz C3 processor.
• The 40× GB2000s fit in a standard VME rack, giving a total of 80 TB of raw capacity.
• The nodes are connected with a 48-port 10/100/1000 switch, dissipating around 3 kW.

Cost and Performance Evaluation

• The cost of the processor, DRAM, ATA disk controller, and power supply is $500.
• The cost of 7,200 RPM PATA drives is $375 × 4.
• The cost of the 48-port 10/100/1000 Ethernet switch is $3,000.
• The average I/O size is 16 KiB for accesses to the historical record and 50 KiB for collecting a new snapshot.
• The performance evaluation is based on the cost per I/O per second (IOPS) of the 80 TB rack.

Storage and I/O Systems

• Non-volatile memory is part of the memory hierarchy system or I/O system, connected to the I/O buses.
• Main choices for storing data are magnetic disks and flash memory.
• Magnetic disks serve as non-volatile storage, are big, cheap, and slow, and reside in the lowest level of the memory hierarchy system.
• Disks have a rotating disk covered with a magnetic surface, using a read-write head per surface to access information.

RAID 6

• RAID 6 is a scheme that allows for recovery of a block even if two disks fail
• It uses p - 1 diagonals to protect p disks
• Each block of the diagonal parity disk contains the parity of the blocks in the same diagonal
• Recovery procedure involves using the row-diagonal approach to recover one of the four blocks within a failed disk, and then using the diagonal parity to recover another block

How RAID 6 Works

• If data disks number 1 and 3 fail, standard RAID recovery using the first row (row parity) does not help
• The recovery procedure starts by recovering one of the four blocks within the failed disk (disk 1), using the diagonal parity
• Then, the diagonal 2 is used to recover a block from disk 1, since it does not involve disk 3
• The loop continues until all blocks are finally recovered

Flash Memory

• Flash memory technology is similar to traditional EEPROM, but with higher memory capacity per chip and low-power consumption
• Read access time is slower than DRAM, but much faster than disks
• A 256-Byte transfer of flash would take around 6.5 µs, and 1,000 times more on disks

RAID Levels

RAID 5

• This level has distributed block-interleaved parity
• It is able to recover from one failure
• Illustrated in Fig. 9.4

RAID 6

• This level is a RAID 5 extension, considering another parity block
• It is able to recover from double failures
• Illustrated in Fig. 9.5

RAID Details

• In RAID 6, there is the row-diagonal parity
• Each diagonal does not cover one disk, i.e., one is left out
• Disks should not fail, but all hardware fails at some point in time

Reliability vs. Availability

• In RAID, reliability becomes a problem considering the system will have less reliability, i.e., more disks then bring a greater fail probability
• Conversely, availability is increased, i.e., failures do not necessarily lead to unavailability
• Latency is not necessarily reduced, availability is enhanced through the addition of redundant disks, and lost information can be rebuilt through redundant data

Standard Levels Summary

• Table 9.1 presents a summary about the RAID levels
• Fig. 9.4 illustrates the RAID 3, RAID 4, and RAID 5 systems