[dm-devel] RHEL6.6 dm-cache and dm-era
heinzm at redhat.com
Thu Sep 25 10:19:55 UTC 2014
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
>>> kernel: device-mapper: era: sb_check failed: magic 1623043: wanted
>>> 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?
> 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>
> 3) Both my metadata_dev and origin_dev are 10G partitions, is this all
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
Would use 8 sector block size (which is tiny!) with disctinct metadata
and data devices (presumabyl your ext4 sits on /dev/mapper/mpathbp1)
> 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 =
> Sep 24 22:44:22 localhost kernel: Oops: 0000 [#1] SMP
> Sep 24 22:44:22 localhost kernel: last sysfs file:
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