Re: RHEL6.6 dm-cache and dm-era

Romu Hu <huruomu@xxxxxxxxx> · Thu, 25 Sep 2014 14:10:40 +0800



    On 2014/9/24 12:10, Brassow Jonathan wrote:

    On Sep 22, 2014, at 7:35 AM, Romu wrote:
      

        I tried the following command to
            set up my era target but the command immediately panics the
            system and the system reboots.
            

            # dmsetup create MyEra --table "0 41941903 era
              /dev/mapper/VG-CacheDataLV_cmeta /dev/mapper/VG-OriginLV
              4096"

            
            The metadata dev and the origin dev are all part of a
              LVM cache LV.  VG-CacheDataLV_cmeta is the cache metadata
              LV on the smaller and faster device, VG-OriginLV is the
              origin LV on the faster and slower device, 41941903 is the
              total sector number of the device of OriginLV (the LV
              takes 100% space of the device), 4096 is the block size of
              OriginLV, I have run 'mkfs.ext4 /dev/mapper/VG-OriginLV'
              before running the dmsetup command.

            
            Below is the message in /var/log/messages after running
              the dmsetup comnmand:
            

            kernel: device-mapper: era: sb_check failed: magic
              1623043: wanted 2126579579

            
              kernel: device-mapper: block manager: superblock
                validator check failed for block 0
              kernel: device-mapper: era: couldn't read_lock
                superblock
            
            
            Any idea?
          

      Sorry, I haven't used dm-era yet.  However, it does appear
        that you are perhaps specifying the wrong devices when creating
        the era device?  Looks like you might be allowing the era target
        and the cache target to use the same metadata area at the same
        time - causing them to corrupt each other's metadata area?
      

       brassow
    
    
    I think the dmsetup command to set up an era target should be
    something like this:

    
    # dmsetup
        create MyEra --table "0 sector_number era metadeta_dev
        origin_dev block_size"

    
    My questions:

    
    1) How to calculate sector_number?  According to
    https://www.kernel.org/doc/Documentation/device-mapper/era.txt, I
    guess it should be (4 * nr_blocks) bytes + buffers, but what is
    nr_blocks and what is buffers?

    2) Any documentation for dmsetup tables?

    3) Both my metadata_dev and origin_dev are 10G partitions, is this
    all right?

    4) My origin_dev is a ext4 filesystem with a block size of 4096, so
    the block_size in the command line should also be 4096?

    
    I tried the following command:

    
    # dmsetup create MyEra --table "0 160000 era
      /dev/mapper/mpathap1 /dev/mapper/mpathbp1 4096"

    
    It immediately panics the kernel (2.6.32-494.el6.i686) with the
    following info in /var/log/messages:

    
    Sep 24 22:44:22 localhost kernel: device-mapper: era: sb_check
      failed: magic 0: wanted 2126579579

    Sep 24 22:44:22 localhost kernel: device-mapper: block
      manager: superblock validator check failed for block 0

    Sep 24 22:44:22 localhost kernel: device-mapper: era:
      couldn't read_lock superblock

    Sep 24 22:44:22 localhost kernel: BUG: unable to handle
      kernel NULL pointer dereference at 00000008

    Sep 24 22:44:22 localhost kernel: IP: [<f7e9a3c7>]
      era_destroy+0x7/0x60 [dm_era]

    Sep 24 22:44:22 localhost kernel: *pdpt = 0000000001945001
      *pde = 00000003ff97f067

    Sep 24 22:44:22 localhost kernel: Oops: 0000 [#1] SMP

    Sep 24 22:44:22 localhost kernel: last sysfs file:
      /sys/module/dm_mod/initstate

    
    Thanks

    Romu

  
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