[linux-lvm] Filesystem corruption with LVM's pvmove onto a PV with a larger physical block size
Stuart D. Gathman
stuart at gathman.org
Thu Feb 28 01:52:04 UTC 2019
On Thu, 28 Feb 2019, Cesare Leonardi wrote:
> Not to be pedantic, but what do you mean with physical block? Because with
> modern disks the term is not always clear. Let's take a mechanical disk with
> 512e sectors, that is with 4k sectors but exposed as 512 byte sectors. Fdisk
> will refer to it with these terms:
> Sector size (logical/physical): 512 bytes / 4096 bytes
> What you are referring as physical size is actually the logical size reported
> by fdisk, right? And if it's correct, I guess that should be safe to add the
> above disk with 512e sectors to an LVM storage composed only by disks with
> real 512 byte sectors. I expect that from the LVM point of view this should
> not be even considered a mixed sector size setup, even if the real physical
> sector size of the added disk is 4096 byte.
> Do you agree or do you think it would be better to test this specific setup?
I would definitely test it, using the same test script that reproduces the
problem with loopback devices.
That said, I believe you are right - it should definitely work. Most of
my drives are 512/4096 logical/phys. If you actually write a single 512
byte sector, however, the disk firmware will have to do a
read/modify/write cycle - which can tank performance.
hdparm will report logical and physical sector size - but there doesn't
seem to be an option to set logical sectory size. There really is no
need once you already support a smaller logical sector size, as the
performance hit can be avoided by aligned filesystems with 4k+ block
size (most modern filesystems).
Once I encountered a bug in drive firmware where the R/M/W did not
work correctly with certain read/write patterns (involving unaligned
multi sector writes). I do not wish that on anyone. (don't worry,
that drive model is long gone...).
Stuart D. Gathman <stuart at gathman.org>
"Confutatis maledictis, flamis acribus addictis" - background song for
a Microsoft sponsored "Where do you want to go from here?" commercial.
More information about the linux-lvm