Re: [linux-lvm] Filesystem corruption with LVM's pvmove onto a PV with a larger physical block size

From: Ingo Franzki <ifranzki@linux.ibm.com>
To: LVM general discussion and development <linux-lvm@redhat.com>,
	Cesare Leonardi <celeonar@gmail.com>
Subject: Re: [linux-lvm] Filesystem corruption with LVM's pvmove onto a PV with a larger physical block size
Date: Fri, 1 Mar 2019 09:05:34 +0100	[thread overview]
Message-ID: <eb3fb7e3-9946-f266-815e-4b49c997e3a4@linux.ibm.com> (raw)
In-Reply-To: <d60d4e1c-ad67-8f3f-b159-13cb3923447f@gmail.com>

On 01.03.2019 02:24, Cesare Leonardi wrote:
> On 28/02/19 09:41, Ingo Franzki wrote:
>> Well, there are the following 2 commands:
>>
>> Get physical block size:
>>   blockdev --getpbsz <device>
>> Get logical block size:
>>   blockdev --getbsz <device>
> 
> I didn't know the blockdev command and, to recap, we have:
> --getpbsz: physical sector size
> --getss: logical sector size
> --getbsz: blocksize used internally by the kernel
> 
> getpbsz/getss correspond to physical/logical sector size reported by fdisk, smartctl, etc.
> 
>> Filesystems seem to care about the physical block size only, not the logical block size.
>>
>> So as soon as you have PVs with different physical block sizes (as reported by blockdev --getpbsz) I would be very careful...
> 
> I've done the test suggested by Stuart and it seems to contradict this.
> I have pvmoved data from a 512/512 (logical/physical) disk to a newly added 512/4096 disk but I had no data corruption. Unfortunately I haven't any native 4k disk to repeat the same test.

Hmm, maybe the size of the volume plays a role as Bernd has pointed out. ext4 may use -b 4K by default on larger devices. 
Once the FS uses 4K block anyway you wont see the problem.

Use  tune2fs -l <device> after you created the file system and check if it is using 4K blocks on your 512/512 device. If so, then you won't see the problem when moved to a 4K block size device.

> 
> Here is what I've done.
> /dev/sdb: SSD with 512/512 sector size
> /dev/sdc: mechanical disk with 512/4096 sector size
> 
> # blockdev -v --getss --getpbsz --getbsz /dev/sdb
> get logical block (sector) size: 512
> get physical block (sector) size: 512
> get blocksize: 4096
> 
> # blockdev -v --getss --getpbsz --getbsz /dev/sdc
> get logical block (sector) size: 512
> get physical block (sector) size: 4096
> get blocksize: 4096
> 
> # pvcreate /dev/sdb4
> # vgcreate vgtest /dev/sdb4
> # lvcreate -L 1G vgtest /dev/sdb4
> # mkfs.ext4 /dev/mapper/vgtest-lvol0
> # mkdir /media/test
> # mount /dev/mapper/vgtest-lvol0 /media/test
> # cp -a SOMEDATA /media/test/
> # umount /media/test
> # fsck.ext4 -f /dev/mapper/vgtest-lvol0
> 
> Filesystem created and no error on it. Now the disk with different physical size:
> 
> # pvcreate /dev/sdc1
> # vgextend vgtest /dev/sdc1
> # pvmove /dev/sdb4 /dev/sdc1
> # fsck.ext4 -f /dev/mapper/vgtest-lvol0
> # mount /dev/mapper/vgtest-lvol0 /media/test
> # ls /media/test/
> 
> The fsck command reports no errors, the filesystem is mountable and the data is there.
> 
> Looks like physical sector size didn't matter here. Or I'm missing something?
> 
> Cesare.
> 
> _______________________________________________
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> read the LVM HOW-TO at http://tldp.org/HOWTO/LVM-HOWTO/
> 
> 

-- 
Ingo Franzki
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