In our quest to... (Oh, man, I always struggle with writing cover letters. But rarely have I become stuck so early on.) Orthogonalize? the block layer (that is, turn hard-coded special options into independent data processing nodes you can put anywhere in your data flow graph), this series adds a copy-on-read filter driver that is supposed to replace the copy-on-read -drive option in the long run. In the short run, it allows you to use COR with blockdev-add.
The patch itself is extremely simple. It's just patch 1. All we have to do for now is to set the BDRV_REQ_COPY_ON_READ flag for read requests. We probably want to extend the COR driver's capabilities later on, though, more on that below under "What to do as a follow-up?". But there is something to do on top of that. One real issue with the current copy-on-read model is that you generally cannot do COR through filter nodes (-drive copy-on-read=on,driver=raw,file.driver=qcow2,...). Unless you are lucky that someone has requested WRITE permission, you will run into a failed assertion from the permission system (see patch 9). Actually, you should always run into such an abort, but the COR code cleverly sneaks its writes around it. Well, actually, the respective assertion is just commented out with a comment noting that we'd need a proper COR filter so we could do the assertion. So the new COR node will take the permission for its child that the current code does not (because it cannot, for the lack of a node that would do this). However, of course we don't want it to require a WRITE permission but just WRITE_UNCHANGED. But when it issues a post-COR write request that goes down to a filter and that filter re-issues that write down to its child, then it will just be a normal write. So suddenly WRITE_UNCHANGED won't be sufficient, we'd need a standard WRITE instead. That isn't what we want, though. So this series also adds a BDRV_REQ_WRITE_UNCHANGED write request flag that is set when issuing a COR write. This flag tells the block code that the WRITE_UNCHANGED permission will be sufficient to execute the request. Filter drivers need to pass this along; format drivers don't because they take a full-on WRITE permission on their file anyway. So most of this series is about adding this new flag. (Note that this does nothing to fix the situation with the old copy-on-read=on. That will still runs into a failed assertion if you poke it the right way. But the cruel reality is that the only way to really fix this is by converting copy-on-read=on into an implicit COR node.) Finally, the test we have for COR (197) is copied and modified to use the COR filter; and another test is added for the situation described above (COR through filter nodes). === What to do as a follow-up? === The obvious thing is to transform the current copy-on-read flag into an implicit COR node. Probably not too difficult, actually (just handle it like snapshot=on), but we need to hide that fact in the query functions. Also, we want to make the stream block job code use this node. Maybe we eventually want this node become the stream block job eventually, actually. That is, give it enough runtime options that you can perform a stream operation just by inserting a COR node into the graph. But probably that won't be possible, because someone still needs to submit reads requests across the whole disk so that everything is actually copied. This could be achieved with a blockdev-copy to null-co://, though. Once both is done, we want to remove the COR code from block.c and move it directly into the COR driver. Max Reitz (9): block: Add COR filter driver block: BLK_PERM_WRITE includes ..._UNCHANGED block: Add BDRV_REQ_WRITE_UNCHANGED flag block: Set BDRV_REQ_WRITE_UNCHANGED for COR writes block/quorum: Support BDRV_REQ_WRITE_UNCHANGED block: Support BDRV_REQ_WRITE_UNCHANGED in filters iotests: Clean up wrap image in 197 iotests: Copy 197 for COR filter driver iotests: Add test for COR across nodes block/Makefile.objs | 2 +- qapi/block-core.json | 5 +- include/block/block.h | 9 ++- block/blkdebug.c | 9 +-- block/blkreplay.c | 3 + block/blkverify.c | 3 + block/cor.c | 173 +++++++++++++++++++++++++++++++++++++++++++++ block/io.c | 12 +++- block/mirror.c | 2 + block/quorum.c | 19 +++-- block/raw-format.c | 9 +-- block/throttle.c | 6 +- tests/qemu-iotests/197 | 1 + tests/qemu-iotests/215 | 120 +++++++++++++++++++++++++++++++ tests/qemu-iotests/215.out | 26 +++++++ tests/qemu-iotests/216 | 117 ++++++++++++++++++++++++++++++ tests/qemu-iotests/216.out | 28 ++++++++ tests/qemu-iotests/group | 2 + 18 files changed, 524 insertions(+), 22 deletions(-) create mode 100644 block/cor.c create mode 100755 tests/qemu-iotests/215 create mode 100644 tests/qemu-iotests/215.out create mode 100755 tests/qemu-iotests/216 create mode 100644 tests/qemu-iotests/216.out -- 2.14.3
