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

[Arno Wagner <arno@wagner.name>]

On Wed, Aug 18, 2010 at 04:47:24PM +0200, Milan Broz wrote:
> 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...

Yes, come to think of it, it is. It is decepticely easy to
do however, since the layers all have clean interfaces.

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

No, I do not think so.

> 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.)

Will try that. Thanks.
 
> > 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.

Good idea. I have not noticed freezes when the focus did
not belong to the VM window. Should check that as well,
maybe it really is CPU 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 :-)

Don't think so. At least not in any obvious manner.

Arno
-- 
Arno Wagner, Dr. sc. techn., Dipl. Inform., CISSP -- Email: arno@wagner.name 
GnuPG:  ID: 1E25338F  FP: 0C30 5782 9D93 F785 E79C  0296 797F 6B50 1E25 338F
----
Cuddly UI's are the manifestation of wishful thinking. -- Dylan Evans

If it's in the news, don't worry about it.  The very definition of 
"news" is "something that hardly ever happens." -- Bruce Schneier