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List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Kevin Wolf , Markus Armbruster , Max Reitz Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On 8/13/19 8:08 PM, Eric Blake wrote: > On 8/13/19 5:44 PM, John Snow wrote: >> This is for the purpose of toggling on/off persistence on a bitmap. >> This enables you to save a bitmap that was not persistent, but may >> have already accumulated valuable data. >> >> This is simply a QOL enhancement: >> - Allows user to "upgrade" an existing bitmap to persistent >> - Allows user to "downgrade" an existing bitmap to transient, >> removing it from storage without deleting the bitmap. >> > > In the meantime, a workaround is: > > create tmp bitmap (non-persistent is fine) > merge existing bitmap into tmp bitmap > delete existing bitmap > recreate original bitmap with desired change in persistence > merge tmp bitmap into re-created original bitmap > delete tmp bitmap > Merge really lets us get away with a lot :) It's a powerful command. And now that merge supports cross-granularities, you can even use it to change the granularity of a bitmap. > (I'm not sure how much, if any of that, has to be done with a > transaction; ideally none, since merging two bitmaps that are both > enabled is not going to lose any bits. And since one of the two ends of > the transaction has a non-persistent bitmap, qemu failing in the narrow > window where the original bitmap does not exist at all is not that much > different from failing while the bitmap is transient. If losing data due > to qemu failure was important, the bitmap should never have been > transient in the first place) > Yup, quite a lengthy workaround, but it _IS_ possible, it's just not very nice. >> Signed-off-by: John Snow >> --- >> >> This is just an RFC because I'm not sure if I really want to pursue >> adding this, but it was raised in a discussion I had recently that it >> was a little annoying as an API design that persistence couldn't be >> changed after addition, so I wanted to see how much code it would take >> to address that. >> >> (So this patch isn't really tested; just: "Hey, look!") >> >> I don't like this patch because it exacerbates my perceived problems >> with the "check if I can make it persistent, then toggle the flag" >> model, where I prefer the "Just try to set it persistent and let it fail >> if it cannot" model, but there were some issues with that patchset that >> I want to revisit. > > The idea itself makes sense. I don't know if libvirt would ever use it, > but it does seem like it could make hand-management of bitmaps easier to > reason about. > Right, this isn't for libvirt as much as it is people doing manual configuration with e.g. qmp-shell (or heaven forbid, their own QMP tooling.) >> +++ b/qapi/block-core.json >> @@ -2001,6 +2001,19 @@ >> 'data': { 'node': 'str', 'name': 'str', '*granularity': 'uint32', >> '*persistent': 'bool', '*autoload': 'bool', '*disabled': 'bool' } } >> >> +## >> +# @BlockDirtyBitmapPersist: > > The QAPI additions look fine to me, regardless of whether you respin the > code based on review there. > Thanks, just wanted this one out there on the record. --js