On 05/31/2017 11:53 AM, Max Reitz wrote: > On 2017-05-31 17:05, Vladimir Sementsov-Ogievskiy wrote: >> 31.05.2017 17:44, Max Reitz wrote: >>> On 2017-05-31 16:29, Vladimir Sementsov-Ogievskiy wrote: >>>> 31.05.2017 16:43, 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. >>>>> >>>>> (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, understand. >>>> >>>> We need to consider also setting in_use flag in the image. We _must not_ >>>> write to image with dirty bitmap, >>>> if in_use flag of this dirty bitmap is not set, as in case of something >>>> fail we will have image with wrong bitmap with >>>> unset in_use flag (which looks ok). >>> Right. >>> >>>> I see two ways to handle it: >>>> >>>> variant 1: >>>> 1. readonly field stays as is (see v19, with normal errors, not only >>>> asserts) >>>> 2. immediately after reopening r/w we do "reopening bitmaps r/w", i.e. >>>> set in_use in the image and set BdrvDirtyBitmap.readonly = false >>>> 3. in reopen_prepare, if reopening r-o do "reopening bitmaps r-o", i.e. >>>> save them into the image and set BdrvDirtyBitmap.readonly = true >>> Sounds good, yes. >>> >>>> variant 2: >>>> 1. instead of 'readonly' add 'dirty' field, set dirty to 0 for all >>>> bitmaps on create >>>> 2. before write/discard check this field in all related bitmaps, and if >>>> dirty=0 (and persistent=1), write IN_USE flag into the image first, set >>>> dirty=1, and only then do write. (if writing IN_USE=1 failed, fail the >>>> whole write) >>>> 3. in reopen_prepare, if reopening r-o do "reopening bitmaps r-o", i.e. >>>> save them into the image and set BdrvDirtyBitmap.dirty = 0 >>> Works, too. >>> >>> I think the second variant would the more "efficient" way (because you >>> only have to flush out dirty dirty bitmaps), but the first one would be >>> simpler and has the great advantage of not requiring a write to the >>> image file when you just want to set a bit in the in-memory dirty >>> bitmap. So I'd personally go for the first variant. >> >> Hmm, why not requiring? Both 1 and 2 do write in_use=1, but (1) do this >> on open/reopen, and (2) before the first write to the image. > > Oh, I didn't read the "before write/discard". Yes, if you check it > before writing, then you won't have to set the flag through > bdrv_set_dirty(). > >> "set a bit in the in-memory..." - are you saying about not-persistent >> dirty bitmaps? In this case, of course, nothing should be written into >> the image, just set dirty=1. > > No, I did mean persistent bitmaps, but bdrv_set_dirty() just sets the > bit in main memory, of course. It only gets written to the image later > (on reopen/close/invalidate). >
There may be some benefit to setting in_use immediately as soon as we admit that we are willing to tolerate writes to the bitmap. It's a performance hit, but it may help on-disk consistency. Or maybe that's a fool's errand? This is a design question we've largely ignored so far, but it's something that will need investigating sooner or later. > Well, your choice. I think both will work. :-) > > Max >
