Re: Error when I compare hashes of export-diff / import-diff

Rafael Diaz Maurin <Rafael.DiazMaurin@xxxxxxxxxxxxxxx> · Wed, 12 Jun 2019 19:02:46 +0200

Le 12/06/2019 à 16:01, Jason Dillaman a écrit :
On Wed, Jun 12, 2019 at 9:50 AM Rafael Diaz Maurin
<Rafael.DiazMaurin@xxxxxxxxxxxxxxx> wrote:
Hello Jason,

Le 11/06/2019 à 15:31, Jason Dillaman a écrit :
4- I export the snapshot from the source pool and I import the snapshot
towards the destination pool (in the pipe)
rbd export-diff --from-snap ${LAST-SNAP}
${POOL-SOURCE}/${KVM-IMAGE}@${TODAY-SNAP} - | rbd -c ${BACKUP-CLUSTER}
import-diff - ${POOL-DESTINATION}/${KVM-IMAGE}
What's the actual difference between the "rbd diff" outputs? There is
a known "issue" where object-map will flag an object as dirty if you
had run an fstrim/discard on the image, but it doesn't affect the
actual validity of the data.

The feature discard=on is activated, and qemu-guest-agent is running on
the guest.

Here are the only differences between the 2 outputs of the "rbd diff
--format plain" (image source and image destination) :
Offset                  Length  Type
121c121
< 14103347200 2097152 data
---
  > 14103339008 2105344 data
153c153
< 14371782656 2097152 data
---
  > 14369685504 4194304 data
216c216
< 14640218112 2097152 data
---
  > 14638120960 4194304 data
444c444
< 15739125760 2097152 data
---
  > 15738519552 2703360 data

And the hashes of the exports are identical (between source and
destination) :
rbd -p ${POOL-SOURCE} export ${KVM-IMAGE}@${TODAY-SNAP} - | md5sum
=> ee7012e14870b36e7b9695e52c417c06

rbd -c ${BACKUP-CLUSTER} -p ${POOL-DESTINATION} export
${KVM-IMAGE}@${TODAY-SNAP} - | md5sum
=> ee7012e14870b36e7b9695e52c417c06

So do you think this can be caused by the fstrim/discard feature.
You said you weren't using fstrim/discard.

In fact, the daemon fstrim is not enabled in the guest, but... in 
proxmox I set the option discard=on and maybe I ran fstrim once by hard 
testing...

If you export both images
and compare those image extents where the diffs are different, is it
just filled w/ zeroes?

I can't find how to to compare if the diffs of the extends are filled by 
w/ zeros.

However I finally succeed getting identical the "rbd diff " hashes with 
the option "--whole-object" like this :

HASH source :
rbd diff --from-snap <YESTERDAY> <POOL>/<IMAGE>@<TODAY> --format json 
--whole-object | md5sum | cut -d ' ' -f 1

Hash destination
rbd -c <cluster-destination.conf> diff --from-snap <YESTERDAY> 
<POOL>/<IMAGE>@<TODAY> --format json --whole-object | md5sum | cut -d ' 
' -f 1

Best regards,
Rafael

--
Rafael Diaz Maurin
DSI de l'Université de Rennes 1
Pôle Infrastructures, équipe Systèmes
02 23 23 71 57

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