On-Disk Caching

On-Disk Caching - Storage Server Storage Cache Database...

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1/31/12 1 On-Disk Caching Method Put RAM on disk controller to cache blocks Seagate ATA disk has .5MB, IBM Ultra160 SCSI has 16MB Some of the RAM space stores “firmware” (an OS) Blocks are replaced usually in LRU order Pros Good for reads if you have locality Cons Expensive Need to deal with reliable writes
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1/31/12 2 Disk Block Caches Main memory rather than disk may hold disk blocks 85% or more of all I/O requests by file system and applications can be satisfied by disk block cache BSD UNIX provides a disk block cache as part of the block-oriented device software layer It consists of between 100 and 1000 individual buffers.
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1/31/12 3 Data Centers Core of enterprise computing A cluster of specialized servers Multiple tiers (SAN = Storage Area Network) SAN Network Storage Database File Applications Email Web
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1/31/12 4 Data Path Database Server Disks SAN
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Unformatted text preview: Storage Server Storage Cache Database Buffer Processors L2 cache Disks, storage server, and database server Widely used caching memory Large access speed gaps Different sizes Various granularity >300 cycles ~125 ns 14 cycles 6 ns ~40 us > 5 ms 1/31/12 5 Two Performance Trends The gaps are increasing large Source: Zhifeng Chen, Optimization of Data Access for Database Applications, PhD Thesis, 2005, UIUC 1/31/12 6 Disk Technology Trends Disks are getting smaller for similar capacity Spin faster, less rotational delay, higher bandwidth Less distance for head to travel (faster seeks) Lighter weight (for portables) Disk data is getting denser More bits/square inch Tracks are closer together Doubles density every 18 months Disks are getting cheaper ($/MB)...
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On-Disk Caching - Storage Server Storage Cache Database...

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