Re: [Libguestfs] [nbdkit PATCH 1/3] server: Add internal support for NBDKIT_FLAG_FAST_ZERO

Qemu was able to demonstrate that knowing whether a zero operation is fast is useful when copying from one image to another: there is a choice between bulk pre-zeroing and then revisiting the data sections (fewer transactions, but depends on the zeroing to be fast), vs. visiting every portion of the disk only once (more transactions, but no time lost to duplicated I/O due to slow zeroes). As such, the NBD protocol is adding an extension to allow clients to request fast failure when zero is not efficient, from servers that advertise support for the new flag. In nbdkit, start by plumbing a new .can_fast_zero through the backends (although it stays 0 in this patch, later patches will actually expose it to plugins and filters to override). Advertise the flag to the client when the plugin provides a .can_fast_zero, and wire in passing the flag down to .zero in the plugin. In turn, if the flag is set and the implementation .zero fails with ENOTSUP/EOPNOTSUPP, we no longer attempt the .pwrite fallback.

+++ b/docs/nbdkit-protocol.pod @@ -173,6 +173,17 @@ This protocol extension allows a client to inform the server about intent to access a portion of the export, to allow the server an opportunity to cache things appropriately. +=item C<NBD_CMD_FLAG_FAST_ZERO> + +Supported in nbdkit E<ge> 1.13.9.