Comment 2 for bug 1793013

I did some more tests today.
Note: this is a mechanical MicroSATA laptop drive.

# WRITE
## 4.15.0-36

$ sync; dd if=/dev/zero of=tempfile bs=1M count=1024; sync
1024+0 registros leídos
1024+0 registros escritos
1073741824 bytes (1,1 GB, 1,0 GiB) copied, 34,2693 s, 31,3 MB/s

$ sync; dd if=/dev/zero of=/tmp/tempfile bs=1M count=1024; sync
1024+0 registros leídos
1024+0 registros escritos
1073741824 bytes (1,1 GB, 1,0 GiB) copied, 21,7465 s, 49,4 MB/s

$ sync; dd if=/dev/zero of=/tmp/tempfile bs=1M count=1024; sync
1024+0 registros leídos
1024+0 registros escritos
1073741824 bytes (1,1 GB, 1,0 GiB) copied, 22,4577 s, 47,8 MB/s

Average: 42,83 MB/s WRITE

## 4.4.0-137

$ sync; dd if=/dev/zero of=/tmp/tempfile bs=1M count=1024; sync
1024+0 registros leídos
1024+0 registros escritos
1073741824 bytes (1,1 GB, 1,0 GiB) copied, 19,0247 s, 56,4 MB/s

$ sync; dd if=/dev/zero of=/tmp/tempfile bs=1M count=1024; sync
1024+0 registros leídos
1024+0 registros escritos
1073741824 bytes (1,1 GB, 1,0 GiB) copied, 20,5483 s, 52,3 MB/s

$ sync; dd if=/dev/zero of=/tmp/tempfile bs=1M count=1024; sync
1024+0 registros leídos
1024+0 registros escritos
1073741824 bytes (1,1 GB, 1,0 GiB) copied, 20,5732 s, 52,2 MB/s

Average: 53,63 MB/s WRITE

# READ
## 4.15.0-36

$ sudo sysctl -w vm.drop_caches=3; dd if=/tmp/tempfile of=/dev/null bs=1M count=1024
1024+0 registros leídos
1024+0 registros escritos
1073741824 bytes (1,1 GB, 1,0 GiB) copied, 24,635 s, 43,6 MB/s

$ sudo sysctl -w vm.drop_caches=3; dd if=/tmp/tempfile of=/dev/null bs=1M count=1024
1024+0 registros leídos
1024+0 registros escritos
1073741824 bytes (1,1 GB, 1,0 GiB) copied, 24,4772 s, 43,9 MB/s

$ sudo sysctl -w vm.drop_caches=3; dd if=/tmp/tempfile of=/dev/null bs=1M count=1024
1024+0 registros leídos
1024+0 registros escritos
1073741824 bytes (1,1 GB, 1,0 GiB) copied, 25,8508 s, 41,5 MB/s

Average: 43 MB/s READ

## 4.4.0-137

$ sudo sysctl -w vm.drop_caches=3; dd if=/tmp/tempfile of=/dev/null bs=1M count=1024
vm.drop_caches = 3
1024+0 registros leídos
1024+0 registros escritos
1073741824 bytes (1,1 GB, 1,0 GiB) copied, 25,4324 s, 42,2 MB/s

$ sudo sysctl -w vm.drop_caches=3; dd if=/tmp/tempfile of=/dev/null bs=1M count=1024
vm.drop_caches = 3
1024+0 registros leídos
1024+0 registros escritos
1073741824 bytes (1,1 GB, 1,0 GiB) copied, 25,1271 s, 42,7 MB/s

$ sudo sysctl -w vm.drop_caches=3; dd if=/tmp/tempfile of=/dev/null bs=1M count=1024
vm.drop_caches = 3
1024+0 registros leídos
1024+0 registros escritos
1073741824 bytes (1,1 GB, 1,0 GiB) copied, 25,0382 s, 42,9 MB/s

Average: 42,6 MB/s READ

We could say that kernel 4.4 and 4.15 read speeds are almost the same, but write speed on kernel 4.15 is about 25% slower; this matters on older machines that use swap, perhaps not so much on higher-end machines relying on SSDs.