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

[Romu Hu <huruomu@gmail.com>]

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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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