Storage Management
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Storage Management
Storage Management
Storage Technology
Views of Data in Query Evaluation
Storage Management
Cost Models
COMP9315 21T1 ♢ Storage Management ♢ [0/10]
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❖ Storage Management
Lowest levels of DBMS related to storage management:
COMP9315 21T1 ♢ Storage Management ♢ [1/10]
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❖ Storage Technology
Persistent storage is
large, cheap, relatively slow, accessed in blocks
used for long-term storage of data
Computational storage is
small, expensive, fast, accessed by byte/word
used for all analysis of data
Access cost HDD:RAM ≅ 100000:1, e.g.
10ms to read block containing two tuples
1µs to compare fields in two tuples
COMP9315 21T1 ♢ Storage Management ♢ [2/10]
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❖ Storage Technology (cont)
Hard disk drives (HDD) are well-established, cheap, high-volume, …
spinning magnetic medium
access requires moving r/w head to position
transfers blocks of data (e.g. 1KB)
Latency: move to track + spin to block = ~10ms (avg)
Volume: one HDD can store up to 20TB (typically 4TB/8TB/…)
Summary: very large, persistent, slow, block-based transfer
COMP9315 21T1 ♢ Storage Management ♢ [3/10]
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❖ Storage Technology (cont)
Solid state drives (SSD) are modern, high-volume devices …
faster than HDDs, no latency
can read single items
update requires block erase then write
over time, writes “wear out” blocks
require controllers that spread write load
Volume: one SSD can store up to 8TB (typically 1TB/2TB/…)
Summary: large, persistent, fast, (partly) block-based transfer
COMP9315 21T1 ♢ Storage Management ♢ [4/10]
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❖ Storage Technology (cont)
Comparison of storage device properties:
RAMHDDSDD
Capacity~ 32GB~ 8TB~ 2TB
Cost/byte~ $10 / GB~ $40 / TB~ $200 / TB
Read latency~ 1µs~ 10ms~ 50µs
Write latency~ 1µs~ 10ms~ 900µs
Read unitbyteblock (e.g. 1KB)byte
Writingbytewrite a blockwrite on empty block
COMP9315 21T1 ♢ Storage Management ♢ [5/10]
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❖ Storage Technology (cont)
Aims of storage management in DBMS:
provide view of data as collection of pages/tuples
map from database objects (e.g. tables) to disk files
manage transfer of data to/from disk storage
use buffers to minimise disk/memory transfers
interpret loaded data as tuples/records
basis for file structures used by access methods
COMP9315 21T1 ♢ Storage Management ♢ [6/10]
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❖ Views of Data in Query Evaluation
COMP9315 21T1 ♢ Storage Management ♢ [7/10]
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❖ Views of Data in Query Evaluation (cont)
Representing database objects during query execution:
DB (handle on an authorised/opened database)
Rel (handle on an opened relation)
Page (memory buffer to hold contents of disk block)
Tuple (memory holding data values from one tuple)
Addressing in DBMSs:
PageID = FileID+Offset … identifies a block of data
where Offset gives location of block within file
TupleID = PageID+Index … identifies a single tuple
where Index gives location of tuple within page
COMP9315 21T1 ♢ Storage Management ♢ [8/10]
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❖ Storage Management
Topics in storage management …
Disks and Files
performance issues and organisation of disk files
Buffer Management
using caching to improve DBMS system throughput
Tuple/Page Management
how tuples are represented within disk pages
DB Object Management (Catalog)
how tables/views/functions/types, etc. are represented
COMP9315 21T1 ♢ Storage Management ♢ [9/10]
<< ∧❖ Cost ModelsThroughout this course, we compare costs of DB operationsImportant aspects in determining cost: data is always transferred to/from disk as whole blocks (pages) cost of manipulating tuples in memory is negligible overall cost determined primarily by #data-blocks read/writtenComplicating factors in determining costs: not all page accesses require disk access (buffer pool) tuples typically have variable size (tuples/page ?)More details later …COMP9315 21T1 ♢ Storage Management ♢ [10/10]Produced: 17 Feb 2021
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