Since you're currently serializing requests, you may be able to manually break up big requests into smaller requests that don't exceed max-transfer, and then wait for the right number of completions with blkioq_do_io(). There is no general limit to the size of memory regions. 32M should always be fine. In particular, using different parts of a single big memory region for many concurrent requests is a reasonable use case. A possible approach is to make nbdkit always allocate page-aligned, file-backed buffers, and attempt to blkio_map_mem_region() them at init time in the libblkio plugin. Note that mapping memory regions even for drivers that don't strictly need them can still improve performance. If this fails and the driver requires mapped memory regions, then one could fall back to bounce buffering with buffers allocated by blkio_alloc_mem_region(). But this probably won't happen in practice, so just failing in this case is probably fine. Alberto