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

[Heinz Mauelshagen <heinzm@redhat.com>]

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Romu,

"dmsetup create MyEra --table "0 20971520 /dev/mapper/mpathbp1 
/dev/mapper/mpathap1 8"

should read

dmsetup create MyEra --table "0 20971520 era /dev/mapper/mpathbp1 
/dev/mapper/mpathap1 8"


Heinz

On 09/25/2014 04:02 PM, Romu wrote:
> 2014-09-25 18:19 GMT+08:00 Heinz Mauelshagen <heinzm@redhat.com 
> <mailto:heinzm@redhat.com>>:
>
>     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 <tel: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
>
>
> Heinz, thank you for your help!
>
> My origin dev is 10G so total sector number is 20971520. 
>  /dev/mapper/mpathbp1 (/dev/dm-6) is the metadata dev, 
> /dev/mapper/mpathap1 (/dev/dm-7) is the origin dev, they are on 
> different LUNs.
>
> I tried the following commands:
>
> # dmsetup create MyEra --table "0 20971520 /dev/mapper/mpathbp1 
> /dev/mapper/mpathap1 8"
> # Target type name /dev/mapper/mpathbp1 is too long.
> # Command failed
>
> # dmsetup create MyEra --table "0 20971520 /dev/dm-6 /dev/dm-7 8"
> # device-mapper: reload ioctl on MyEra failed: Invalid argument
> # Command failed
>
> And when executing the second command I see the following in 
> /var/log/messages:
>
> Sep 25 06:37:48 localhost kernel: device-mapper: table: 253:8: 
> /dev/dm-6: unknown target type
> Sep 25 06:37:48 localhost kernel: device-mapper: ioctl: error adding 
> target to table
> Sep 25 06:37:48 localhost multipathd: dm-8: remove map (uevent)
> Sep 25 06:37:48 localhost multipathd: dm-8: devmap not registered, 
> can't remove
> Sep 25 06:37:48 localhost multipathd: dm-8: remove map (uevent)
> Sep 25 06:37:48 localhost multipathd: dm-8: devmap not registered, 
> can't remove
>
> Then I run 'mkfs.ext4 /dev/dm-6' and tried the second command again 
> but got the same result.
>
> Any idea?
>
> Thanks
> Romu
>
>
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