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Storage Management
Storage Management
Storage Technology
Views of Data in Query Evaluation Storage Management
Cost Models
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❖ Storage Management
Lowest levels of DBMS related to storage management:
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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 elds in two tuples
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❖ Storage Technology (cont)
Hard disk drives (HDD) are well-established, cheap, high-volume,
…
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
spinning magnetic medium
access requires moving r/w head to position transfers blocks of data (e.g. 1KB)
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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
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❖ Storage Technology (cont) Comparison of storage device properties:
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RAM
HDD
SDD
Capacity
~ 32GB
~ 8TB
~ 2TB
Cost/byte
~$10/GB
~$40/TB
~$200/TB
Read latency
~ 1μs
~ 10ms
~ 50μs
Write latency
~ 1μs
~ 10ms
~ 900μs
Read unit
byte
block (e.g. 1KB)
byte
Writing
byte
write a block
write on empty block
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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 les manage transfer of data to/from disk storage
use buffers to minimise disk/memory transfers interpret loaded data as tuples/records
basis for le structures used by access methods
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❖ Views of Data in Query Evaluation
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<< ∧ >> ❖ Views of Data in Query Evaluation (cont)
Representing database objects during query execution:
DB (handleonanauthorised/openeddatabase)
Rel (handleonanopenedrelation)
Page (memorybuffertoholdcontentsofdiskblock) Tuple (memoryholdingdatavaluesfromonetuple)
Addressing in DBMSs:
PageID = FileID+Offset…identi esablockofdata
where Offset gives location of block within le TupleID = PageID+Index…identi esasingletuple
where Index gives location of tuple within page
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❖ Storage Management Topics in storage management …
Disks and Files
performance issues and organisation of disk les 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
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❖ Cost Models
Throughout this course, we compare costs of DB operations Important 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/written
Complicating factors in determining costs:
not all page accesses require disk access (buffer pool) tuples typically have variable size (tuples/page ?)
More details later …
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Produced: 17 Feb 2021
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