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Questions and Answers
What is the primary advantage of NAND flash memories over NOR flash memories?
What is the primary advantage of NAND flash memories over NOR flash memories?
Higher storage density
What is the maximum number of recordings allowed for flash memory before wear-out, depending on the manufacturer?
What is the maximum number of recordings allowed for flash memory before wear-out, depending on the manufacturer?
100,000 to 1 million
What is the primary purpose of the BIOS in computer systems?
What is the primary purpose of the BIOS in computer systems?
Basic input/output system
What is the main difference between NOR and NAND flash memories in terms of read access?
What is the main difference between NOR and NAND flash memories in terms of read access?
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What is the main reason for the replacement of hard drives in mobile systems with solid-state disks (SSDs)?
What is the main reason for the replacement of hard drives in mobile systems with solid-state disks (SSDs)?
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What is the main advantage of flash memory over traditional EEPROM?
What is the main advantage of flash memory over traditional EEPROM?
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What is the primary limitation of flash memory in terms of its lifespan?
What is the primary limitation of flash memory in terms of its lifespan?
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What is the primary factor that affects the cost of flash memory?
What is the primary factor that affects the cost of flash memory?
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What is the main difference between flash memory and traditional hard drives in terms of cost?
What is the main difference between flash memory and traditional hard drives in terms of cost?
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What is the main purpose of evaluating the performance, cost, and dependability of a system designed to provide high I/O performance?
What is the main purpose of evaluating the performance, cost, and dependability of a system designed to provide high I/O performance?
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What is the typical range of tracks per disk surface in a magnetic disk?
What is the typical range of tracks per disk surface in a magnetic disk?
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How does the disk access information in a sector?
How does the disk access information in a sector?
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What is the main purpose of magnetic disks in a computer system?
What is the main purpose of magnetic disks in a computer system?
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What is the typical range of sectors per track in a magnetic disk?
What is the typical range of sectors per track in a magnetic disk?
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How do magnetic disks store data?
How do magnetic disks store data?
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What is the significance of the cylinder-head-sector (CHS) addressing in magnetic disks?
What is the significance of the cylinder-head-sector (CHS) addressing in magnetic disks?
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What is the role of the arm assembly in magnetic disks?
What is the role of the arm assembly in magnetic disks?
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What is the typical characteristic of magnetic disks in terms of their cost, size, and speed?
What is the typical characteristic of magnetic disks in terms of their cost, size, and speed?
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What is the significance of the read-write head in magnetic disks?
What is the significance of the read-write head in magnetic disks?
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What is the relationship between magnetic disks and the I/O system?
What is the relationship between magnetic disks and the I/O system?
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Study Notes
RAID 6
- RAID 6 is a scheme that can recover 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
- The row-diagonal approach is used to recover failed disks
- It starts by recovering one of the four blocks within the failed disk, and then uses the diagonal parity to recover another block
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, compared to 1,000 times more on disks
Storage Overview
- Non-volatile memory can be viewed as part of the memory hierarchy system or as part of the I/O system
- The main choices for storing data are magnetic disks and flash memory
- Magnetic disks are big, cheap, and slow, and reside in the lowest level of the memory hierarchy system
Magnetic Disks
- Magnetic disks are used for non-volatile (persistent) storage
- Disks are based on a rotating disk covered with a magnetic surface, with a read-write head per surface to access information
- Disks may have more than one platter, and use a cylinder-head-sector (CHS) addressing system
- The sector is the smallest unit that can be addressed in a disk, and has a physical size that can vary depending on the track and sector
Magnetic Disks Performance
- Seek time is the time to position the arm over the addressed track, and is generally between 5 to 12 ms
- Rotational latency is the waiting time for the desired sector to rotate under the read-write head, and is about 3,600 to 15,000 RPM
- The actual seek time can be only 25% to 33% of the time disclosed by manufacturers due to locality of disk reference
RAID
- RAID (Redundant Array of Independent Disks) is a system that allows for multiple simultaneous accesses since the data are spread into multiple disks
- Two basic techniques are used: stripping, where sequential data is logically allocated on separate disks to increase performance, and mirroring, where data is copied to identical disks to increase availability
- RAID systems can be categorized into different levels, including RAID 0, RAID 1, RAID 2, RAID 3, RAID 4, and RAID 5
- Each level has its own characteristics, such as redundancy, availability, and performance
Flash Memory Types
- NOR flash memories were a direct competitor of traditional EEPROM, and were randomly addressable and typically used in the computer's BIOS
- NAND flash memories have higher storage density, but can only be read in blocks, and are much cheaper per gigabyte and more common than NOR flash
- The price of flash memories has decreased over time, from 2/GiBin2010to2/GiB in 2010 to 2/GiBin2010to0.3/GiB in 2016
Magnetic Disks Performance
- Average Rotational Latency (ARL) is the average latency to the desired information, halfway around the disk, and can be computed as 0.5 × Rotation Period.
- Rotation Period is calculated by dividing 60 by the disk's RPM (revolutions per minute).
- Common values for disk rotational speed/latency are 5,400 and 7,200 RPM.
Transfer Time
- Transfer time depends on transfer size per sector, rotational speed, and recording density (bits/inch).
- Typical transfer rates are from 3 to 65 MiB/s.
Magnetic Disks Evolution
- Magnetic disks evolved over the years, with an increase in disk density, a steep price reduction, and a considerable increase in RPM.
- RPM increased from 3,600 RPM (in the '80s) to close to 15,000 RPM (2000's), but does not continue to increase due to problems with high rotational speed.
Disk Access Time
- Disk Access Time (DAT) is computed as the sum of Seek Time, Rotational Latency, Transfer Time, Controller Time, and Queuing Delay.
RAID Systems
- Disks are non-volatile and the lowest level in the memory hierarchy.
- The sector is the smallest unit that can be addressed in a disk.
- Earlier, all tracks had the same number of sectors, but currently, disks have tracks with different numbers of sectors to increase storage capacity.
Seek Time
- Seek Time is the time to position the arm over the addressed track and is generally between 5 to 12 ms.
- The actual Seek Time can be only 25% to 33% of the time disclosed by manufacturers due to locality with respect to the disk reference.
Rotational Latency
- Rotational Latency is the waiting time for the desired sector to rotate under the read-write head and is about 3,600 to 15,000 RPM, i.e., 16 ms to 4 ms per revolution.
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Description
This quiz describes the RAID 6 configuration, where even if two disks fail, data can be recovered. It explains the concept of row parity and diagonal parity disks.
