[dm-crypt] aes-xts-plain with aes_x86_64 makes my SSD 5x slower than my encrypted HD

Marc MERLIN marc at merlins.org
Sun Jul 22 21:07:58 CEST 2012


I got a new Samsumg 830 512GB SSD which is supposed to be very high
performance.
The raw device seems fast enough on a quick hdparm test:
/dev/sda4:
 Timing cached reads:   14258 MB in  2.00 seconds = 7136.70 MB/sec
 Timing buffered disk reads: 1392 MB in  3.00 seconds = 463.45 MB/sec <<<<

which is 4x faster than my non encrypted spinning disk, as expected.


But once I encrypt it, it drops to 5 times slower than my 1TB spinning
disk in the same laptop:
gandalfthegreat:~# hdparm -tT /dev/mapper/ssdcrypt 
/dev/mapper/ssdcrypt:
 Timing cached reads:   15412 MB in  2.00 seconds = 7715.37 MB/sec
 Timing buffered disk reads:  70 MB in  3.06 seconds =  22.91 MB/sec <<<<

gandalfthegreat:~# hdparm -tT /dev/mapper/cryptroot (spinning disk)
/dev/mapper/cryptroot:
 Timing cached reads:   16222 MB in  2.00 seconds = 8121.03 MB/sec
 Timing buffered disk reads: 308 MB in  3.01 seconds = 102.24 MB/sec <<<<

I used aes-xts-plain as recommended on
http://www.mayrhofer.eu.org/ssd-linux-benchmark

gandalfthegreat:~# cryptsetup status /dev/mapper/ssdcrypt
/dev/mapper/ssdcrypt is active.
  type:    LUKS1
  cipher:  aes-xts-plain
  keysize: 256 bits
  device:  /dev/sda4
  offset:  4096 sectors
  size:    926308752 sectors
  mode:    read/write

I tried 
cryptsetup luksFormat --align-payload=8192 
the first time, so my offset was 8K, but that did not make a
difference in speed.

gandalfthegreat:~# lsmod |grep -e aes
aesni_intel            50443  66 
cryptd                 14517  18 ghash_clmulni_intel,aesni_intel
aes_x86_64             16796  1 aesni_intel

Kernel: 3.4.4-amd64

gandalfthegreat:~# cryptsetup --version
cryptsetup 1.4.3

I know that SSDs are weird and all, but getting a raw device speed of a
mere 23MB/sec down from 463MB/s and compared to 102MB/s for a similarly
spinning drive, is a problem, is it not?

Any suggestions would be appreciated.

Thanks,
Marc
-- 
"A mouse is a device used to point at the xterm you want to type in" - A.S.R.
Microsoft is to operating systems ....
                                      .... what McDonalds is to gourmet cooking
Home page: http://marc.merlins.org/


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