Am 07.05.2021 um 09:11 hat Vladimir Sementsov-Ogievskiy geschrieben: > 17.03.2021 20:15, Alberto Garcia wrote: > > When the x-blockdev-reopen was added it allowed reconfiguring the > > graph by replacing backing files, but changing the 'file' option was > > forbidden. Because of this restriction some operations are not > > possible, notably inserting and removing block filters. > > > I now started to work on making backup-top filter public.. > > And I think, we'll need separate commands to insert/remove filters > anyway.. As blockdev-reopen has the following problems: > > 1. It can't append filter above top node, connected to block-device. > (but bdrv_append() can do it)
We once had some patches that made the 'drive' qdev property runtime writable. What happened to them? > 2. It can't simultaneously create new node and append it. This is > important for backup-top filter, which unshares write even when has no > writing parent. Now bdrv_append() works in a smart way for it: it > first do both graph modification (add child to filter, and replace > original node by filter) and then update graph permissions. So, we'll > need a command which in one roll create filter node and inserts it > where needed. What backup-top could do, however, is enabling restrictions only if it has a parent (no matter whether that parent requires writing or not). > 3. blockdev-reopen requires to specify all options (otherwise, they > will be changed to default). So it requires passing a lot of > information. But we don't need to touch any option of original bs > parent to insert a filter between parent and bs. In other words, we > don't want to reopen something. We want to insert filter. Yeah, but this was a decision we made consciously because otherwise we'd have a hard time telling which options should be updated and which shouldn't, and we would need separate QAPI types for open and reopen. If we now say that this is a reason for avoiding blockdev-reopen even though changing some option is the goal, that would be inconsistent. > > === > > Hmm, another mentioned use of blockdev-reopen was reopening some RO > node to RW, to modify bitmaps.. And here again, blockdev-reopen is not > very convenient: > > 1. Again, it requires to specify all options (at least, that was not > default on node open).. And this only to change one property: > read-only. Seems overcomplicated. > > 2. Bitmaps modifications are usually done in transactions. Adding a > clean transaction support for small command that reopens only to RW, > and back to RO on transaction finalization seems simpler, than for > entire generic blockdev-reopen. > > > === > > Hmm, interesting. x-blockdev-reopen says that not specified options > are reset to default. x-blockdev-reopen works through > bdrv_reopen_multiple, so I think bdrv_reopen_mutliple should reset > options to default as well. Now look at bdrv_reopen_set_read_only(): > it specifies only one option: "read-only". This means that all other > options will be reset to default. But for sure, callers of > bdrv_reopen_set_read_only() want only to change read-only status of > node, not all other options. Do we have a bug here? The difference between these cases is the keep_old_opts parameter to bdrv_reopen_queue(). It is false for x-blockdev-reopen, but true in bdrv_reopen_set_read_only(). Kevin