Re: Multicore Support?

Chris wrote:
> Curious things to note here about md raid.  When using encryption on top
> of an device mapper device (md device), is inheritly slow and should
> show no improvement for multiple threads doing encryption.  Multiple
> threads could only help improve speeds on multiple independent devices.

I am not sure if I understand it correctly...

If you create multiple encrypted devices, you have multiple threads
now already.

md is not device-mapper device, it is old raid device, but yes, it combines
multiple disc into one block device,

If there is running multiple IO on *one* device and they are independent
we can encrypt/decrypt them in parallel - that should be implemented.
(The problem is in order and synchronization.)

>    The device mapper creates an abstraction that pretends multiple
> devices are one (for raid at least) so dm-crypt could not take advantage
> of this due to block encryption.
see above.

>  If you encrypt multiple raw devices
> then make a device mapper on them, you would notice improvements in
> speed.  Unfortunately this doesn't work properly, it is an unstable
> configuration (not sure why, I think both use the same area of the
> device to store metadata).

The md raid over multiple dm-crypt devices is strange configuration,
I can imagine that there will be problem when some device fails.
You need then not only to use mdadm to remove failed raid device, but also
remove dm-crypt mapping for underlying device, it will not happen automatically.
But in principle it should work.

Milan
--
mbroz@...