Re: [libvirt RFCv11 00/33] multifd save restore prototype

On 10/11/23 16:05, Daniel P. Berrangé wrote: > > Instead of using 'getfd' though we have to use 'add-fd'. > > Anyway, this lets us do FD passing as normal, whle also > letting us specify the offset. > > {"execute": "add-fd", "arguments": {"fdset-id":"migrate"}} > {"execute": "migrate", "arguments": {"detach":true,"blk":false,"inc":false,"uri":"file:/dev/fdset/migrate,offset=124456"}}' > >> Internally, the QEMU multifd code just reads and writes using pread, pwrite, so there is in any case just one fd to worry about, >> but who should own it, libvirt or QEMU? > > How about both :-) I need to familiarize a bit with this, there are pieces I am missing. Can you correct here? OPTION 1) libvirt opens the file and has the FD, writes the header, marks the offset, then we dup the FD in libvirt for the benefit of QEMU, optionally set the flags of the dup to "O_DIRECT" (the usual case) depending on --bypass-cache, pass the duped FD to QEMU, QEMU does all the pread/pwrite on it with the correct offset (since it knows it from the file:// URI optional offset parameter), then libvirt closes the duped fd libvirt rewrites the header using the original fd (needed to update the metadata), libvirt closes the original fd OPTION 2) libvirt opens the file and has the FD, writes the header, marks the offset, then we pass the FD to QEMU, QEMU dups the FD and sets it as "O_DIRECT" depending on a passed parameter, QEMU does all the pread/pwrite on it with the correct offset (since it knows it from the file:// URI optional offset parameter), QEMU closes the duped FD, libvirt rewrites the header using the original fd (needed to update the metadata), libvirt closes the original fd I don't remember if QEMU changes for the file offsets optimization are already "block friendly" ie they operate correctly whatever the state of O_DIRECT or ~O_DIRECT, I think so. They have been thought with O_DIRECT in mind.

So I would tend to see OPTION 1) as more attractive as QEMU does not need to care about another parameter, whatever has been chosen in libvirt in terms of bypass cache is handled in libvirt.