On 11/18/19 3:16 PM, Cole Robinson wrote: > On 10/25/19 4:28 AM, Patrik Martinsson wrote: >> Hi Tim, >> >> I recently stumbled on the same thing, accidentally shrinking a blockdevice. >> >> I have written a patch for virsh that will force the user to append a >> '--force' flag if shrinking is desired. >> >> The behavior is somewhat still inconsistent with the vol-resize >> command, however a bigger rewrite is needed to make both commands >> operate exactly the same, which I don't know if actually needed. >> >> Previous discussion can be found below, >> - https://www.redhat.com/archives/libvir-list/2019-October/msg00258.html >> - https://www.redhat.com/archives/libvir-list/2019-October/msg01437.html >> >> Best regards, >> Patrik >> >> >> On Thu, Oct 24, 2019 at 6:04 PM Tim Small <tim@xxxxxxxxxxx> wrote: >>> >>> Hello, >>> >>> virsh has two commands which can be used to resize block devices - >>> "blockresize" for volumes in use by and active guest, and "vol-resize" >>> for volumes which are not in use. >>> >>> The vol-resize syntax allows to specify the size as a delta (increase or >>> decrease vs. the current size), and also refuses to shrink a volume >>> unless the "--shrink" argument is also passed. >>> >>> Most other tools which can be used for block device resizing (outside of >>> libvirt) also have similar "--shrink" argument requirements when >>> reducing the size of an existing block device. e.g. ceph requires >>> "--allow-shrink" when using the "rbd resize" command. >>> >>> The lack of such a safety device makes "blockresize" a foot-gun (which I >>> recently found to great effect when I typoed the domain name to another >>> valid domain). >>> >>> It seems I am not alone in making this error e.g. >>> https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=902171 >>> >>> One possible solution would be to make a new command e.g. "domblkresize" >>> or perhaps "live-resize", which implement the "--shrink" and "--delta" >>> behaviour to make it consistent with "vol-resize" syntax, and mark the >>> "blockresize" command as deprecated in the documentation and help (so >>> that existing automation which depends on the current behaviour doesn't >>> break). >>> >>> Any thoughts? Should I open this as an RFE? >>> > > Considering there's been multiple people hitting it, I think it's > something we should fix in libvirt. Just need buy in from other devs. To > summarize: > > 'virsh blockresize' will online resize an image path for a running VM. > It does this with the qemu block_resize monitor command via the > virDomainBlockResize API. The API doesn't provide any protection against > shrinking the disk image though, which I presume is both the less common > intention of the operation, and much less often safe to do for a running > VM. And a user typo can mean data loss > > virsh vol-resize, which is storage API virStorageVolResize, is for > offline image resizing, mostly using qemu-img. It has had a SHRINK API > flag from the outset, rejecting requests to reduce the image size unless > the flag is passed. Seems like a safe pattern to follow. > > Can we change existing blockresize behavior? I think it's reasonable; > we've added flags to other APIs that are required to restore old > behavior, UNDEFINE_NVRAM for one example. > I brought this question up in this thread: https://www.redhat.com/archives/libvir-list/2019-December/msg00817.html danpb suggested making this a protection that lives in virsh only. So, change blockresize to reject shrinking, but add a --shrink option to override that behavior, and all the code lives in tools/ so the old API behavior is preserved. You can CC me on a patch and I'll review it (but I'll be offline until January) Thanks, Cole -- libvir-list mailing list libvir-list@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/libvir-list
