Setup: SSD encrypted with luks aes-xts 512 bits (256 bits AES key), ext4 filesystem
dd write performance of 138 MB/s, CPU usage 97-100 %
dd if=/dev/zero of=testfile status=progress bs=32M count=128 4294967296 bytes (4,3 GB, 4,0 GiB) copied, 31 s, 138 MB/s 128+0 Datensätze ein 128+0 Datensätze aus 4294967296 bytes (4,3 GB, 4,0 GiB) copied, 31,0463 s, 138 MB/s
dd read performance 110 MB/s, CPU usage starts with > 90 %, then falls to about 50-60 % and then at the end of file reading goes up to > 90 % again.
#crop cache before sudo sh -c "echo 1 > /proc/sys/vm/drop_caches"
Now do the test:
dd if=testfile of=/dev/null status=progress bs=32M 4261412864 bytes (4,3 GB, 4,0 GiB) copied, 39 s, 109 MB/s 128+0 Datensätze ein 128+0 Datensätze aus 4294967296 bytes (4,3 GB, 4,0 GiB) copied, 39,1345 s, 110 MB/s
Now lets take a closer look at samba:
Writing a 1 GB file to a samba share at disk benchmarked above gives about 73 MB/s, CPU usage is only about 70 %.
Reading a 1 GB file from samba share gives only about 64 MB/s, CPU usage is about 55 %. Also watch this graph: It starts slowly and then speed goes up and down, generating some kind of wave form.
Copying this file immediately again, when it is in cache, then it is copied with 112 MB/s, so full GigabitEthernet speed as it should.
Compare to unencrypted drive:
dd write speed 133 MB/s
dd read speed 207 MB/s
Samba write: 112 MB/s
Samba read: 112 MB/s
So LUKS encryption alone gives sufficient speed, Samba alone also has sufficient speed. In combination there is a huge performance drop while there are still plenty of CPU ressources available which are used when solely dd is used.
What is wrong here? Why isn't CPU fully used when doing operations for samba while it is with dd? What can be done to have also full performance / CPU usage with smb and luks encryption?