Linux – get current hard disk read write in ubuntu and make log file

Because you can't compare both of them. sar -b is at specific times, at an interval of 10 minutes or whatever you have specified in your cron job.

But /proc/diskstats is a kernel run time value. It is not calculated at any specific time interval but more of a counter that kernel keeps for accounting. It is from where iostat reads the data. If you specify iostat -xdk 5 5 then, at 5 seconds interval iostat is going to read the /proc/diskstats file and get the weighted average of data from there. Important point to note is that /proc/diskstats is not time dependent and hence you cannot compare it with sar -b which runs at fixed, particular intervals in time.

If you want a more elaborate description, I can check the code and tell but now it is really late in night here.

You can do that by combining loop devices and device mapper, thanks to "everything is a file". But with one limitation: The file size cannot be changed (as you cannot increase block devices by writing to them). So if you want to append data then you have to create a bigger block device and overwrite the appended dummy data.

# for testing, Create 4 files
echo foo | dd of=block0 cbs=512 conv=block
echo bar | dd of=block1 cbs=512 conv=block
echo bat | dd of=block2 cbs=512 conv=block
echo baz | dd of=block3 cbs=512 conv=block
# Create a loop device for each of the 4 files
losetup /dev/loop0 block0
losetup /dev/loop1 block1
losetup /dev/loop2 block2
losetup /dev/loop3 block3
# Create a device map named "test" using those loop devices
(
    echo "0 1 linear /dev/loop0 0"
    echo "1 1 linear /dev/loop1 0"
    echo "2 1 linear /dev/loop2 0"
    echo "3 1 linear /dev/loop3 0"
) | dmsetup create test
$EDITOR /dev/mapper/test # use overwrite mode only

For extending the file you may create a big sparse file and use that as additional device.