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

[Heinz Mauelshagen <heinzm@redhat.com>]

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On 09/25/2014 08:10 AM, Romu Hu wrote:
> 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?

No calculation: it's the size of your era target in sectors. Typically 
"blockdev --getsz origin_dev"

> 2) Any documentation for dmsetup tables?

Look for examples in the kernel source trees Documentaion/device.mapper.

table line syntax is: "start_sector length_in_sectors <target> 
<target_arguments{0,N}>"

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

Metadata device size is plenty but that depends on how many eras with 
how many updates you want to have.

> 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?

You may use 4096 = _8_ sectors, you used 4096 sectors = 2MB below.
It's the granularity the era target housekeeps blocks for.

>
> I tried the following command:
>
> # dmsetup create MyEra --table "0 160000 era /dev/mapper/mpathap1 
> /dev/mapper/mpathbp1 4096"

May not be  same physical device for data and metadata!
If it is, thta'd explain your oops.

# dmsetup create MyEra --table "0 $(blockdev --getsz 
/dev/mapper/mpathbp) era whatever_disctinct_metadata_device 
/dev/mapper/mpathbp1 8"

Would use 8 sector block size (which is tiny!) with disctinct metadata 
and data devices (presumabyl your ext4 sits on /dev/mapper/mpathbp1)

Heinz


>
> 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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