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EdiD
Asked: 2017-02-19 07:39:39 +0800 CST

Understanding IO Block size

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hdparm -I /dev/sda output: 

Logical  Sector size:                   512 bytes
Physical Sector size:                   512 bytes

stat somefile output: 

Size: 509           Blocks: 8          IO Block: 4096   regular file

Why IO Block is 4096 ? Isn't it the same as physical sector size which is 512 bytes ?

filesystem

1 Answers

  1. Short answer... generalization...

    In the old days, with small capacity, slow hard disk drives:

    logical sector = physical sector = IO block = 512 bytes

    In current days, with high capacity, fast hard/SSD disk drives:

    logical sector = physical sector = 512 bytes, IO block = 4096

    Drives with IO block size of 4096 are called "Advanced Format" drives. Any read request will always read 1 block minimum (or 8 sectors). This way, sector 1 is ready to process, and sector 2-8 are ready, if need be, without a slow disk read for the next block(s). This speeds up overall data transfers, and keeps block numbers at a manageable number.

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