[linux-lvm] Mixing devices with different logical or physical block size in oVirt LVM based storage
snitzer at redhat.com
Mon Feb 4 16:25:27 UTC 2019
On Sat, Feb 02 2019 at 6:54pm -0500,
Nir Soffer <nsoffer at redhat.com> wrote:
> We working on enabling 4k block size in oVirt block storage domain, built
> using VG
> on multipath devices on shared storage.
> We have incomplete support for 4k, added in 2011, for this bug:
> When creating or extending a VG, we check that all PVs are using same
> logical and
> phyisical block size, and we store both logical and physical block size in
> the VG tags.
> We get the block sizes from
> We also enforce that device physical block size is not smaller than
> logical block size,
> This check was added in this patch, trying to enable block size != 512.
> There is no
> explanation in the patch or in the review comments why we need to validate
> When we start to use a VG, we validate that all the devices are using the
> stored logical
> and physical block size.
> In vdsm itself, we use the logical block size to manage vdsm metadata,
> assuming that writing
> and reading one block of logical block size bytes is atomic, and we can
> read and write
> different blocks from different hosts at the same time.
> The relevant code validating PV block sizes is here:
> Reading the comments in bug 732980, I don't see anything about physical
> block size. It looks
> like this is unnecessary check, and we should check only the logical block
> Regarding mixing devices with different logical block size, according to
> We should not extend an LV over devices with different block size, as this
> will change the device
> logical block size (e.g change from 512 to 4k), and the change may break
> the upper layer that
> already use the device and assume the previous logical block size.
This idea that 4K writes to a 512b physical drive aren't going to be
atomic, and that that is going to be the basis for some upper level
failure is handwaving and overly paranoid TBH.
> Based on this, I think we are ok with limiting VG to devices with same
> logical block size, so any
> LV can be extended to any device.
> I think this code should change to:
> 1. When creating a VG, check that all PVs use the same logical block size
> 2. Store the logical block size in the VG tag
> 3. When extending the VG, check that the new PVs use the same logical
> block size
> 4. When starting to use a VG, check that stored logical block size matches
> PVs logical block size
> What do you think?
I think you shouldn't care. Or please show me a case where all this
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