As currently written, structured reply is documented as: > NBD_REPLY_TYPE_OFFSET_DATA (1) > > This chunk type is in the content chunk category. length MUST be at least 9. It represents the contents of length - 8 bytes of the file, starting at the absolute offset from the start of the export. which implies that the data size must be non-zero. But clients can request a read of size 0 (the spec doesn't forbid it, but neither does it define special semantics for it), and the existing qemu implementation as of qemu commit f140e300 sends NBD_REPLY_TYPE_OFFSET_DATA with length of 8 and no data payload if the client requests a 0-byte read. Should we specifically allow this particular answer, or should a 0-length read be answered solely by NBD_REPLY_TYPE_NONE, meaning that qemu's current behavior needs a tweak? Either way, I probably need another tweak to the NBD spec for structured reads. -- Eric Blake, Principal Software Engineer Red Hat, Inc. +1-919-301-3266 Virtualization: qemu.org | libvirt.org
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