Calculate the MD5 checksum of a file over and over, clearing the disk cache between each read, to try to identify read errors that are happening on a hard drive. Ref: https://www.pommepause.com/2016/02/the-case-of-the-dying-hard-drive-that-flipped-bits/

test_checksum.php

<?php

$file = '/mnt/hdd5/persistent-app-data/downloading/vvgoor1586DSFGKL.part10.rar';

while (TRUE) {
    exec("sync ; sudo sh -c 'echo 3 > /proc/sys/vm/drop_caches'");
    $data1 = file_get_contents($file);
    exec("sync ; sudo sh -c 'echo 3 > /proc/sys/vm/drop_caches'");
    $data2 = file_get_contents($file);

    $md5_data1 = md5($data1);
    $md5_data2 = md5($data2);
    if ($md5_data1 != $md5_data2) {
        echo "First read MD5: $md5_data1\n";
        echo "Second read MD5: $md5_data2\n";
        for ($i=0; $i<strlen($data1); $i++) {
            if ($data1[$i] != $data2[$i]) {
                echo "Wrong byte at pos $i: 0x" . dechex(ord($data1[$i])) . " vs 0x" . dechex(ord($data2[$i])) . "\n";
            }
        }
    } else {
        echo "OK\n";
    }
}