[dm-crypt] Slow I/O with LUKS on amd64

Milan Broz mbroz at redhat.com
Wed Mar 9 16:46:32 CET 2011


On 03/09/2011 03:38 PM, Carlo Wood wrote:
> On Wed, 09 Mar 2011 11:21:53 +0100
> Milan Broz <mbroz at redhat.com> wrote:
> 
> Actually, I have been trying to use dmcrypt for my swap
> (anything that is swapped HAS to be encrypted, of course,
> because it can be anything).
> 
> However, while the disk benchmarked gives me the normal
> 50 MB/s for that disk/partition, reading/writing the swap
> with encryption only gives me around 1 MB/s. This was
> so unacceptable that I turned off swap all together:
> I rather crash when running out of memory than having to
> wait minutes every moment of the day (and for some reason
> linux likes to use the swap a lot, even while normally
> it is not running out of memory).
> 
> I have an Intel Core 2(tm) cpu (a Quad Core Extreme QX6700),
> but also x86_64; stock debian kernel.

It could be another problem with swap. I remeber such
reports some time ago, maybe it is back.

Anyway, I am using Debian on x86_64 as well (but with recompiled
upstream kernel) - all above ext3/ext4/LVM over dmcrypt
for my stable machine and see no such problems.

How did you test swap device access?
If you can reproduce it, please provide more info.

See for example this my system (Debian testing, except upstream kernel,
dmcrypt uses AES-NI), just plain dd from swap volume over LUKS:

# echo 3 > /proc/sys/vm/drop_caches

# dd if=/dev/notebook/swap of=/dev/null bs=4k
1024000+0 records in
1024000+0 records out
4194304000 bytes (4.2 GB) copied, 42.0784 s, 99.7 MB/s

# lsblk /dev/sda2
NAME                       MAJ:MIN RM   SIZE RO MOUNTPOINT TYPE
sda2                         8:2    0  87.8G  0            part
└─sda2_crypt (dm-0)        253:0    0  87.8G  0            CRYPT
  ├─notebook-data (dm-1)   253:1    0  68.3G  0 /home      LVM
  ├─notebook-swap (dm-2)   253:2    0   3.9G  0 [SWAP]     LVM
  └─notebook-root64 (dm-3) 253:3    0  15.6G  0 /          LVM

(lsblk is from new util-linux, I am just testing some new things here)

Milan


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