Lecture 10- Physical Storage Flashcards
What are each of these levels?
- External level = the way data is seen by individual users
- Conceptual/Logical level = views database as a collection of tables in the relational model
- Internal/Physical = the way data is stored inside the system
Storage classification based on?
- Speed of data access
- Cost per unit of data
- Reliability of the medium
What is primary, secondary and tertiary storage?
- Primary = cache and main memory
- Secondary = flash memory (SSD) and magnetic disk (HDD)
- Tertiary = magnetic tapes and optical disks
As we move down the hierarchy, what happens?
As we move down the hierarchy, the cost per bit decreases, whereas the access time increases
What are some parts of physical disk structure?
- Disk surface = divided into tracks
- Sectors = tracks are subdivided into sectors (the smallest unit of information that can be read of written to the disk
Sectors are typically what size?
512 bytes
What is the key to lower I/O cost?
Reduce seek/rotation delays
What are some software solutions to reducing I/O costs?
- Arrange blocks of a file sequentially on disk = store pages containing related information close together on disk, page size trade off
- Read/Write in bigger chunks
- Buffering = keep cache of recently accessed pages in main memory
What is the page size trade-off?
- Large page size = data related to x stored in same page hence additional page transfer can be avoided
- Small page size = reduce transfer time, reduce buffer size in main memory
Describe the hardware solution to reduce I/O costs
- Arrange disk arrays = several disks that give abstraction of a single, large disk
- Partition data into stripping units and distribute them over several disks
- A disk array refers to a collection of multiple physical hard drives that are combined ot work as a single, larger logical drive
- The idea is to aggregate the storage capacity and performance of several disks
More disks == ?
More failures
What are the main measure of disk quality?
- Capacity
- Access time
- Data transfer rate
- Reliability
What is access, transfer and seek time?
- Access time = seek time + rotational delay + transfer time
- Transfer time = time to actually read or write the data in the block
- Seek time = time for repositioning the arm over the correct track
What is rotational delay?
Time spent waiting for the sector to be accessed to appear under the head
What is the difference between sequential access and random access?
- Sequential access = successive requests are for successive block numbers on the same track/adjacent tracks (seek may be required for the first block, successive requests would either not require a seek or require a seek to adjacent tacks)
- Random access = successive requests are for tracks that are randomly located on disk (each request requires a seek)
