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read-ahead.md

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Disk Readahead

Get the readahead value

$ blockdev --report
RO    RA   SSZ   BSZ   StartSec            Size   Device
...
rw   256  4096  4096          0     57847840768   /dev/nvme0n1
rw   256  4096  4096       2048     28923400192   /dev/nvme0n1p1
rw   256  4096  4096   56494080     28922871808   /dev/nvme0n1p2
rw   256   512  4096          0    256060514304   /dev/sdc
rw   256   512  4096       2048    256059465728   /dev/sdc1
rw   256   512  4096          0   1000204886016   /dev/sda
rw   256   512  4096       2048   1000203091968   /dev/sda1
rw   256   512  4096          0    256060514304   /dev/sdb
rw   256   512  4096       2048    128028684800   /dev/sdb1
rw   256   512  4096  250058752    128030433280   /dev/sdb2
  • RA: the size of readahead in sectors (512 bytes)
    • e.g., The readahead size: RA * sector size = 256 * 512 = 131,072 bytes = 128 KB

Set the readahead value

$ sudo blockdev --setra 2048 /dev/nvme0n1
  • --setra: set readahead (in 512-byte sectors)
    • e.g., --setra 2048: set the readahead value to 1MB (1 * 1024 * 1024 / 512 = 2048)