On Sat, Feb 02 2019 at 6:54pm -0500, Nir Soffer <nsoffer@xxxxxxxxxx> 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: > [1]https://bugzilla.redhat.com/732980 > 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 > /sys/block/dm-X/queue/{logical,physical}_block_size. > 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 > this. > > [2]https://github.com/oVirt/vdsm/commit/7e79153705891a91a06eb31cd642fb209d10ff86 > 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: > > [3]https://github.com/oVirt/vdsm/blob/8b043e402f41d8a82b9f832be5f582b8520b38bc/lib/vdsm/storage/lvm.py#L1110 > 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 > size. > Regarding mixing devices with different logical block size, according to > [4]https://bugzilla.redhat.com/show_bug.cgi?id=732980#c8 > 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 concern matters. Mike _______________________________________________ linux-lvm mailing list linux-lvm@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/linux-lvm read the LVM HOW-TO at http://tldp.org/HOWTO/LVM-HOWTO/
