Hello Keith, On Wed, May 21, 2025 at 03:31:02PM -0700, Keith Busch wrote: > From: Keith Busch <kbusch@xxxxxxxxxx> > > I was never happy with previous block copy offload attempts, so I had to > take a stab at it. And I was recently asked to take a look at this, so > here goes. > > Some key implementation differences from previous approaches: > > 1. Only one bio is needed to describe a copy request, so no plugging > or dispatch tricks required. Like read and write requests, these > can be artbitrarily large and will be split as needed based on the > request_queue's limits. The bio's are mergeable with other copy > commands on adjacent destination sectors. > > 2. You can describe as many source sectors as you want in a vector in > a single bio. This aligns with the nvme protocol's Copy implementation, > which can be used to efficiently defragment scattered blocks into a > contiguous destination with a single command. > > Oh, and the nvme-target support was included with this patchset too, so > there's a purely in-kernel way to test out the code paths if you don't > have otherwise capable hardware. I also used qemu since that nvme device > supports copy offload too. In order to test this series, I wrote a simple user space program to test that does: 1) open() on the raw block device, without O_DIRECT. 2) pwrite() to a few sectors with some non-zero data. 3) pread() to those sectors, to make sure that the data was written, it was. Since I haven't done any fsync(), both the read and the write will from/to the page cache. 4) ioctl(.., BLKCPY_VEC, ..) 5) pread() on destination sector. In step 5, I will read zero data. I understand that BLKCPY_VEC is a copy offload command. However, if I simply add an fsync() after the pwrite()s, then I will read non-zero data in step 5, as expecting. My question: is it expected that ioctl(.., BLKCPY_VEC, ..) will bypass/ignore the page cache? Because, as far as I understand, the most common thing for BLK* operations is to do take the page cache into account, e.g. while BLKRESETZONE sends down a command to the device, it also invalidates the corresponding pages from the page cache. With that logic, should ioctl(.., BLKCPY_VEC, ..) make sure that the src pages are flushed down to the devices, before sending down the actual copy command to the device? I think that it is fine that the command ignores the data in the page cache, since I guess in most cases, you will have a file system that is responsible for the sectors being in sync, but perhaps we should document BLKCPY_VEC and BLKCPY to more clearly highlight that they will bypass the page cache? Which also makes me think, for storage devices that do not have a copy command, blkdev_copy_range() will fall back to __blkdev_copy(). So in that case, I assume that the copy ioctl actually will take the page cache into account? Kind regards, Niklas
