sudo rm -rf /*

This is an example of using dd to asynchronously write 30% of memory to disk which would happen to be affected by the change in vm.dirty_ratio:

root # MEMTOTAL_MBYTES=`free -m | grep Mem: | awk '{print $2}'`
root # sysctl vm.dirty_ratio=40
root # dd if=/dev/zero of=zerofile ibs=1048576 count=$((MEMTOTAL_MBYTES*30/100))
2507145216 bytes (2.5 GB) copied, 8.00153 s, 313 MB/s
root # sysctl vm.dirty_ratio=20
dd if=/dev/zero of=zerofile ibs=1048576 count=$((MEMTOTAL_MBYTES*30/100))
2507145216 bytes (2.5 GB) copied, 10.1593 s, 247 MB/s

Note that the parameter affects the time it takes for the command to complete and the apparent write speed of the device. With dirty_ratio=40, more of the data is cached and written to disk in the background by the kernel. It is very important to note that the speed of I/O is identical in both cases. To demonstrate, this is the result when dd synchronizes the data before exiting:

root # sysctl vm.dirty_ratio=40
root # dd if=/dev/zero of=zerofile ibs=1048576 count=$((MEMTOTAL_MBYTES*30/100)) conv=fdatasync
2507145216 bytes (2.5 GB) copied, 21.0663 s, 119 MB/s
root # sysctl vm.dirty_ratio=20
root # dd if=/dev/zero of=zerofile ibs=1048576 count=$((MEMTOTAL_MBYTES*30/100)) conv=fdatasync
2507145216 bytes (2.5 GB) copied, 21.7286 s, 115 MB/s

Note that dirty_ratio had almost no impact here and is within the natural variability of a command. Hence, dirty_ratio does not directly impact I/O performance but it may affect the apparent performance of a workload that writes data asynchronously without synchronizing

www.tgoop.com/sudo_rmrf/307

2.6K viewsApr 8, 2022 at 07:12

This is an example of using dd to asynchronously write 30% of memory to disk which would happen to be affected by the change in vm.dirty_ratio:

root # MEMTOTAL_MBYTES=`free -m | grep Mem: | awk '{print $2}'`
root # sysctl vm.dirty_ratio=40
root # dd if=/dev/zero of=zerofile ibs=1048576 count=$((MEMTOTAL_MBYTES*30/100))
2507145216 bytes (2.5 GB) copied, 8.00153 s, 313 MB/s
root # sysctl vm.dirty_ratio=20
dd if=/dev/zero of=zerofile ibs=1048576 count=$((MEMTOTAL_MBYTES*30/100))
2507145216 bytes (2.5 GB) copied, 10.1593 s, 247 MB/s

root # sysctl vm.dirty_ratio=40
root # dd if=/dev/zero of=zerofile ibs=1048576 count=$((MEMTOTAL_MBYTES*30/100)) conv=fdatasync
2507145216 bytes (2.5 GB) copied, 21.0663 s, 119 MB/s
root # sysctl vm.dirty_ratio=20
root # dd if=/dev/zero of=zerofile ibs=1048576 count=$((MEMTOTAL_MBYTES*30/100)) conv=fdatasync
2507145216 bytes (2.5 GB) copied, 21.7286 s, 115 MB/s

BY sudo rm -rf /*