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CSI 2132 > Chapter 12: Physical Storage Systems > Flashcards

Chapter 12: Physical Storage Systems Flashcards

1
Q

def: volatile storage

A

loses contents when power is switched off

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2
Q

def: non-volatile storage

A

contents persist even when power is switched off

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3
Q

storage hierarchy (6)

A
  • cache
  • main memory
  • flash memory
  • magnetic disk
  • optical disk
  • magnetic tapes
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4
Q

def: primary storage

A
  • cache and main memory
  • fastest media, but volatile
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5
Q

def: secondary storage

A
  • flash memory and magnetic disks
  • non-volatile, moderately fast access time (AKA on-line storage)
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6
Q

def: tertiary storage

A
  • non-volatile, slow access time
  • AKA off-line storage
  • used for archival storage
  • magnetic tape and optical storage
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7
Q

disk interface standard families (3)

A

SATA (Serial ATA)
- SATA 3 supports data transfer speeds of up to 6 gigabits/sec

SAS (Serial Attached SCSI)
- SAS Version 3 supports 12 gigabits/sec

NVMe (Non-Volatile Memory Express) interface
- Works with PCIe connectors to support lower latency and higher
transfer rates
- Supports data transfer rates of up to 24 gigabits/sec

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8
Q

def: Storage Area Networks (SAN)

A

a large number of disks are connected
by a high-speed network to a number of servers

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9
Q

def: n Network Attached Storage (NAS)

A

networked storage provides a file
system interface using networked file system protocol, instead of
providing a disk system interface

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10
Q

def: sectors

A
  • smallest unit of data that can be read or written
  • sector size typically 512 bytes
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11
Q

size of tracks

A

Typical sectors per track: 500 to 1000 (on inner tracks) to 1000 to
2000 (on outer tracks)

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12
Q

Cylinder

A

i consists of i
th track of all the platters

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13
Q

def: Disk controller

A

– interfaces between the computer system and the disk
drive hardware
accepts high-level commands to read or write a sector
- initiates actions such as moving the disk arm to the right track and
actually reading or writing the data
- Computes and attaches checksums to each sector to verify that
data is read back correctly
- If data is corrupted, with very high probability stored checksum
won’t match recomputed checksum
- Ensures successful writing by reading back sector after writing it
- Performs remapping of bad sectors

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14
Q

def: access time

A
  • the time it takes from when a read or write request is
    issued to when data transfer begins
  • consists of seek time, rotational latency
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15
Q

def: Seek time

A

– time it takes to reposition the arm over the correct track

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16
Q

def: Rotational latency

A

time it takes for the sector to be accessed to
appear under the head

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17
Q

def: Data-transfer rate

A

rate at which data can be retrieved from or stored
to the disk

18
Q

def: Disk block

A

a logical unit for storage allocation and retrieval

Smaller blocks: more transfers from disk

Larger blocks: more space wasted due to partially filled blocks

19
Q

def: Sequential access pattern

A
  • Successive requests are for successive disk blocks
  • Disk seek required only for first block
20
Q

def: Random access pattern

A
  • Successive requests are for blocks that can be anywhere on disk
  • Each access requires a seek
  • Transfer rates are low since a lot of time is wasted in seeks
21
Q

def: I/O operations per second (IOPS)

A
  • Number of random block reads that a disk can support per second
  • 50 to 200 IOPS on current generation magnetic disks
22
Q

def: Mean time to failure (MTTF)

A

average time the disk is expected to
run continuously without any failure, typically 3-5 years

23
Q

def: NAND flash

A
  • used widely for storage, cheaper than NOR flash
  • requires page-at-a-time read (Not much difference between sequential and random read)
  • Page can only be written once (Must be erased to allow rewrite)
24
Q

def: Solid state disks

A

Use standard block-oriented disk interfaces, but store data on multiple
flash storage devices internally

25
Q

def: erase block

A

erase happens

26
Q

def: Remapping

A

logical page addresses to physical page addresses avoids
waiting for erase

27
Q

Flash translation table purpose

A
  • tracks mapping
  • stored in label field of flash page
  • carried out by flash translation layer
28
Q

def: Hybrid disks

A

combine small amount of flash cache with larger magnetic
disk

29
Q

def: RAID: Redundant Arrays of Independent Disks

A

disk organization techniques that manage a large numbers of disks, providing a view of a single disk of

  • high capacity and high speed and high reliability
30
Q

def: redundancy

A

store extra information that can be used to rebuild
information lost in a disk failure

31
Q

Mirroring / shadowing

A

Duplicate every disk. Logical disk consists of two physical disks.
* Every write is carried out on both disks
 Reads can take place from either disk
* If one disk in a pair fails, data still available in the other
 Data loss would occur only if a disk fails, and its mirror disk also
fails before the system is repaired
* Probability of combined event is very small
 Except for dependent failure modes such as fire or building
collapse or electrical power surges

32
Q

Mean time to data loss depends on ——–,
and ——— to repair

A
  • mean time to failure
  • mean time
33
Q

Two main goals of parallelism in a disk system

A
  1. Load balance multiple small accesses to increase throughput
  2. Parallelize large accesses to reduce response time
34
Q

Bit-level striping

A
  • split the bits of each byte across multiple disks
  • seek/access time worse than for a single disk
  • not used much any more
35
Q

Block-level striping

A

– with n disks, block i of a file goes to disk (i mod n)+ 1
- Requests for different blocks can run in parallel if the blocks reside on
different disks
- A request for a long sequence of blocks can utilize all disks in parallel

36
Q

RAID Level 0

A
  • Block striping; non-redundant
  • Used in high-performance applications where data loss is not critical.
37
Q

RAID Level 1

A
  • Mirrored disks with block striping
  • Offers best write performance
38
Q

Parity blocks

A
  • Parity block j stores XOR of bits from block j of each disk
  • When writing data to a block j, parity block j must also be computed
    and written to disk
  • More efficient for writing large amounts of data sequentially
39
Q

RAID Level 5

A

Block-Interleaved Distributed Parity; partitions data and
parity among all N + 1 disks, rather than storing data in N disks and parity in 1 disk

Block writes occur in parallel if the blocks and their parity blocks are
on different disks

40
Q

RAID Level 6

A

P+Q Redundancy scheme; similar to Level 5, but stores
two error correction blocks (P, Q) instead of single parity block to guard
against multiple disk failures.
* Better reliability than Level 5 at a higher cost
 Becoming more important as storage sizes increase

CSI 2132 (6 decks)

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