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

Ingo Franzki ifranzki at linux.ibm.com
Fri Mar 1 07:59:13 UTC 2019

On 01.03.2019 04:41, Stuart D. Gathman wrote:
> On Fri, 1 Mar 2019, Cesare Leonardi wrote:
>> 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.
> Use a loopback device with logical block size set to 4096 to confirm
> that your test does detect corruption (using the same LV, filesystem,
> data).
> I believe by "physical sector", the original reporter means logical,
> as he was using an encrypted block device that was virtual - there
> was no "physical" sector size.  It was "physical" as far as the
> file system was concerned - where "physical" means "the next layer
> down".
Well, let me cite from https://www.saout.de/pipermail/dm-crypt/2019-February/006078.html from Ondrej Kozina which is also referenced in my original post:

"dm-crypt advertise itself as a block device with physical sector size 
*at least* equal to encryption sector size. Traditionally it's been only 
512B. So classical dm-crypt mapped over device with phys. sector size = 
512B has no impact. If you mapped dm-crypt over block device with native 
physical sector size = 4096 you got dm-crypt exposing same limits as 
underlying block device. Again no problem. Just internally dm-crypt 
performed encryption on 512B blocks, but it had no impact on exposed limits.

But things get a bit different with encryption sector size > 512B.

If you map dm-crypt with encryption sector size set to 4096B over block 
device with physical sector size = 512B, dm-crypt must increase device 
limits to 4096. Because when it does encryption it must be aligned to 
4096 bytes (and same wrt minimal i/o size)."

> Indeed, even the rotating disk drives make the physical sector size
> invisible except to performance tests.  SSD drives have a "sector" size
> of 128k or 256k - the erase block, and performance improves when aligned
> to that.

Ingo Franzki
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