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Asked: 2026-06-04T00:20:48+00:00

I have following code for opening all files: int ret= open(zFile, flags, mode); posix_fadvise

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I have following code for opening all files: 

int ret= open(zFile, flags, mode);
posix_fadvise (ret, 0, 0, POSIX_FADV_RANDOM);

and posix_fadvise return 0 means success.

But it is not working correctly.

OS: Ubuntu 12.04

Strace is showing that the read done by the program is
29088 bytes.

I used following command to calculate read from strace log 

cat fadvise3.log | grep read | awk '{SUM+=$NF} END {print SUM}'

But iotop is showing that program has read about 2.5 MB.

  1. Is there a way I can know why it is reading that much ?
  2. Why iotop O/p is not matching with strace?
  3. Why posix_fadvise (ret, 0, 0, POSIX_FADV_RANDOM); is not working?
  4. How can I disable read ahead for a process?

More details about read:
I am using sqlite library. and modified their function posixOpen to open a database. Read is done by sqlite.

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1 Answer

  1. Editorial Team

    Disk read are done in disk block, this is a hardware limit, POSIX_FADV_RANDOM can’t change that. If you read less then a block, you still have to pay the cost of that block.

    Even so, posix_fadvise, as its name suggest, is just an advise to the kernel. A hint. Not a strict rule.

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