On 2017-06-02 00:29, John Snow wrote: > > > 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?)
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...) I don't mind either way. I like a dirty flag because this is a common concept (we basically cache the persistent bitmaps in RAM, so it's natural for them to have a dirty/clean state); but I also like keeping the read-only flag because it's probably simpler to implement (and makes just as much sense). Max
signature.asc
Description: OpenPGP digital signature