[dm-devel] Is there a way to see updated contents of a DM target's underlying device while the DM target is in use?

[Riches Jr, RobertX M]

Many thanks to those who responded with helpful suggestions.  A couple of the suggestions were sufficient to make my test script behave as I expect--to liberate the data that had been stuck in the page cache twilight zone.  :-)

For the benefit of anyone who might come across the archives with a similar problem, here's what worked and what didn't, with all actions taken between writing and reading:

  - Using 'dd -iflag=direct' to read from /dev/vg1/lv1 (the underlying device) worked.

  - Doing 'blockdev --flushbufs /dev/vg1/lv1' (on the underlying device) worked.

  - Doing 'blockdev --flushbufs /dev/mapper/fred01' (on the device exposed by dm-linear or my module) did not solve the problem.

  - Doing 'dmsetup suspend fred01' did not work.

  - Doing fsync() on /dev/mapper/fred01 did not work.

  - Doing fsync() on /dev/vg1/lv1 did not work.

Of those above that "worked", each was independently sufficient to solve the problematic symptoms. IOW, I only had to apply one solution, not all of them.

Again, thanks for all the solutions.

Robert Riches
Azad Consultant at Intel
 
-----Original Message-----
From: Alasdair G Kergon [mailto:agk at redhat.com] 
Sent: Wednesday, June 30, 2010 5:52 PM
To: Riches Jr, RobertX M
Cc: Dm-Devel Mailing List Send-To Address (dm-devel at redhat.com)
Subject: Re: [dm-devel] Is there a way to see updated contents of a DM target's underlying device while the DM target is in use?

On Wed, Jun 30, 2010 at 04:53:23PM -0700, Riches Jr, RobertX M wrote:
> I didn't see any queues or caches in dm-linear that would need to be flushed.
 
It's your test script that is not behaving as you expect:)

Add conv=odirect to dd

Add blockdev --flushbufs after other writes.

Alasdair