Re: [dm-crypt] dm-crypt flush-to-disk freezes

[Milan Broz <mbroz@redhat.com>]

On 08/18/2010 04:12 PM, Arno Wagner wrote:
> On Wed, Aug 18, 2010 at 01:43:13PM +0200, Milan Broz wrote:
>> On 08/17/2010 08:46 PM, Arno Wagner wrote:
>>> The set-up is a dm-crypt partition with a Windows XP 
>>> VM and the current VMplayer, all with kernel 2.6.34.4
>>> from kernel.org and current vmware-tools in the VM.
>>
>> Can you be more specific?
>>
>> You have direct partition with LUKS and this is mapped
>> to vmware directly as disk, right?
>> No fs in the middle?
> 
> This is VMwarePlayer. I do not think it even supports putting
> the OS on a partition. So no, there is ext3 in between and the
> VM filesystem Image goes into several 2GB files.
>  
> Ah, forgot one possibly important detail: This is dm-crypt on top
> of md RAID1. 

This is quite complex setup...
(vmware ws suppported direct access to disk device, it is probably
better setup here. Did they this functionality from player?)

I think ext3 part is problem here too, is it using barriers?
(Try to switch them off, it is mount option - just to see if it helps.)

> Low latency desktop. But the freezes are too long to 
> be CPU related and there is heavy disk activity when 
> they happen. Therefore mty conclusion that this is
> an emegerncy flush.

There can be more problems, dm-crypt itself contains cond_resched()
after every encrypted sector allowing decision of scheduler
to switch there. So dm-crypt istelf should not freeze host system this
way, but it doesn't mean that VM suffers from starvation.

I'll check this, I am using similar system, maybe it uncovers some
other problems...

> Anyways, thanks and I think I will just run the OS unencrypted
> for the moment. The data still goes into the encrypted partition
> and curiously that does not cause problems. Very strange. 

(There is also option to encrypt VM in vmware workstation7 directly.
I wonder if they support it in player :-)

Milan