On 05/31/2017 09:43 AM, Max Reitz wrote: > On 2017-05-30 08:50, Vladimir Sementsov-Ogievskiy wrote: >> Thank you for this scenario. Hmm. >> >> So, as I need guarantee that image and bitmap are unchanged, >> bdrv_set_dirty should return error and fail the whole write. Ok? > > I don't know. That would mean that you couldn't commit to an image that > has a persistent auto-loading bitmap, which doesn't seem very nice to me. > > I'm not quite sure what to do myself. So first I'd definitely want the > commit operation to succeed. That means we'd have to automatically make > the bitmap non-readonly once we write to it. The "readonly" flag would > then be an "unchanged" flag, rather, to signify that the bitmap has not > been changed since it was loaded, which means that it does not need to > be written back to the image file. > > Now the issue remains that if you modify a persistent bitmap that is > stored in an image file that is opened RO when it's closed, you won't be > able to write the modifications back. > > So in addition, I guess we'd need to "flush" all persistent bitmaps > (that is, write all modifications back to the file and set the > "unchanged" flag (you could also call it "dirty" and then mean the > opposite) for each bitmap) not only when the image is closed or > invalidated, but also when it is reopened read-only. >
Makes sense. > (block-commit reopens the backing BDS R/W, then writes to them, thus > modifying the dirty bitmaps, and finally reopens the BDS as read-only; > before that happens, we will have to flush the modified bitmap data.) > OK, so it would perhaps be enough to toggle the RO flag on/off when nodes get reopened. When they get reopened RO, we'd need to flush at that point. (Right?) Of course, a changed flag makes this a little moot as it is probably more flexible; but there is something slightly attractive about the more rigid form. (Hmm, for the purposes of periodic flushing, we may want a changed flag anyway...) > Max > Thanks for the scenario and the explainer.
