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Re: [MISC] Disk IO?



Jesper Krogh wrote:
Når man læser noget fra disk, hvad er så den mindste IO-størrelse? Er
det en "block"? eller en "byte"? eller et word?


Den mindste IO enhed er en block, som for et ext3 filsystem er 4096 bytes. Prøv 'tune2fs -l device'. Ved oprettelsen af filsystemet med mkfs.ext3 kan block size sættes til 1024, 2048 eller 4096 bytes per block.
Eksemplet er at man har 2 stykker data om sammen ting. Den ene del A
bruges meget ofte den anden del B bruges knap så tit.


Hvis den mindste enhed er en blok på 1024 bytes og der kan være 10*(A+B)
i den, så giver det ikke mening at lave 20 enkelte filer..
Størrelsen af (A+B) er altså ca. 100 bytes eller 50 bytes for hver A og B. 20 filer A og B med 50 bytes i hver optager således 20 K diskplads, selvom der reelt kun er 100 bytes data. Det er altså en masse "ubenyttet" disk; det kaldes vistnok intern fragmentering.


Håber jeg fik forklaret problemet godtnok.


Jesper



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