Chapter 12: Physical Storage Systems Flashcards
def: volatile storage
loses contents when power is switched off
def: non-volatile storage
contents persist even when power is switched off
storage hierarchy (6)
- cache
- main memory
- flash memory
- magnetic disk
- optical disk
- magnetic tapes
def: primary storage
- cache and main memory
- fastest media, but volatile
def: secondary storage
- flash memory and magnetic disks
- non-volatile, moderately fast access time (AKA on-line storage)
def: tertiary storage
- non-volatile, slow access time
- AKA off-line storage
- used for archival storage
- magnetic tape and optical storage
disk interface standard families (3)
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
def: Storage Area Networks (SAN)
a large number of disks are connected
by a high-speed network to a number of servers
def: n Network Attached Storage (NAS)
networked storage provides a file
system interface using networked file system protocol, instead of
providing a disk system interface
def: sectors
- smallest unit of data that can be read or written
- sector size typically 512 bytes
size of tracks
Typical sectors per track: 500 to 1000 (on inner tracks) to 1000 to
2000 (on outer tracks)
Cylinder
i consists of i
th track of all the platters
def: Disk controller
– 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
def: access time
- the time it takes from when a read or write request is
issued to when data transfer begins - consists of seek time, rotational latency
def: Seek time
– time it takes to reposition the arm over the correct track
def: Rotational latency
time it takes for the sector to be accessed to
appear under the head
def: Data-transfer rate
rate at which data can be retrieved from or stored
to the disk
def: Disk block
a logical unit for storage allocation and retrieval
Smaller blocks: more transfers from disk
Larger blocks: more space wasted due to partially filled blocks
def: Sequential access pattern
- Successive requests are for successive disk blocks
- Disk seek required only for first block
def: Random access pattern
- 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
def: I/O operations per second (IOPS)
- Number of random block reads that a disk can support per second
- 50 to 200 IOPS on current generation magnetic disks
def: Mean time to failure (MTTF)
average time the disk is expected to
run continuously without any failure, typically 3-5 years
def: NAND flash
- 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)
def: Solid state disks
Use standard block-oriented disk interfaces, but store data on multiple
flash storage devices internally
