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List:       evms-devel
Subject:    Re: [Evms-devel] Snapshot failing after kernel upgrade (more
From:       Thomas Zeitlhofer <thomas.zeitlhofer () nt ! tuwien ! ac ! at>
Date:       2006-08-04 11:10:25
Message-ID: 20060804111025.GA21333 () swan ! nt ! tuwien ! ac ! at
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Hello Kevin,

> 1. I create a snapshot.
> 2. I create the volume.
> 3. I successfully backup the data from the first volume.
> 4. I delete the volume and snapshot.
> 5. I then try to create a second snapshot but see the error: "Floating
> point exception"

my system is also Debian Etch with evms 2.5.5 and I experienced the same
problems when upgrading form 2.6.16.x to 2.6.17.x. 

While your patch targets the divide-by-zero bug, it seems that a more
serious problem is caused by evms when removing a snapshot. The dm-*
devices that are created during a snapshot are not removed properly and
it seems that the structure found in /sys/block/dm-* is in an
inconsistant state afterwards, which also breaks "dmsetup ls --tree".  
Here is an example:

 Setup: 
 
 - LVM2 storage container on top of RAID1 
 - storage regions "data" and "snap"
 - logical volume "data"
 
 - logical volume "data" encrypted via dm-crypt -> /dev/mapper/cryptdata
 
 
 1.) no snapshot:
 ----------------
 
 analyzing /sys/block/dm-* (arrows point from dm-* to its slaves):
 
  dm-16 -> md254
  dm-21 -> dm-3 -> dm-2 -> md254
 
 # dmsetup ls --tree
 
   lvm2|raid|snap (253:16)
    `- (9:254)
  
   cryptdata (253:21)
    `-data (253:3)
       `-lvm2|raid|data (253:2)
          `- (9:254)
 
 
 2.) active snapshot (snapshotted volume /dev/evms/data):
 --------------------
 
 analyzing /sys/block/dm-* (arrows point from dm-* to its slaves):
 (created devices: dm-23, dm-24, dm-25)
 
  dm-25 -> dm-24 -> dm-16 -> md254
        \_____________________
                              |
                              V 
  dm-21 -> dm-3 -> dm-2 -> dm-23 -> md254
 
 # dmsetup ls --tree
 
   snap (253:25)
    |-snap#sibling# (253:24)
    |  `-lvm2|raid|snap (253:16)
    |     `- (9:254)
    `-lvm2|raid|data (253:23)
       `- (9:254)
  
   cryptdata (253:21)
    `-data (253:3)
       `-lvm2|raid|data#origin# (253:2)
          `-lvm2|raid|data (253:23)
             `- (9:254)
  
 
 3.) removed snapshot:
 ----------------
 
 analyzing /sys/block/dm-* (arrows point from dm-* to its slaves):
 (removed devices: dm-24, dm-25)
 (note: dm-23 has not been removed)
 
  dm-16 -> md254
  dm-23 
        \____________
                     |
                     V
  dm-21 -> dm-3 -> dm-2 -> md254
 
 # dmsetup ls --tree
   device-mapper: deps ioctl failed: No such device or address
   _deps: task run failed for (253:3)
   Command failed
 
 # dmsetup ls 
  
   lvm2|raid|snap  (253:16)
   lvm2|raid|data  (253:2)
   data            (253:23) 
   cryptdata       (253:21)

Note, the device "/dev/evms/data" has changed from 253:3 to 253:23 but
253:3 is still found in the structure under /sys/block/.

Regards,

 Thomas

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