On Mon, Jun 17, 2024 at 03:57:27PM -0300, Fabiano Rosas wrote: > When multifd is used along with mapped-ram, we can take benefit of a > filesystem that supports the O_DIRECT flag and perform direct I/O in > the multifd threads. This brings a significant performance improvement > because direct-io writes bypass the page cache which would otherwise > be thrashed by the multifd data which is unlikely to be needed again > in a short period of time. > > To be able to use a multifd channel opened with O_DIRECT, we must > ensure that a certain aligment is used. Filesystems usually require a > block-size alignment for direct I/O. The way to achieve this is by > enabling the mapped-ram feature, which already aligns its I/O properly > (see MAPPED_RAM_FILE_OFFSET_ALIGNMENT at ram.c). > > By setting O_DIRECT on the multifd channels, all writes to the same > file descriptor need to be aligned as well, even the ones that come > from outside multifd, such as the QEMUFile I/O from the main migration > code. This makes it impossible to use the same file descriptor for the > QEMUFile and for the multifd channels. The various flags and metadata > written by the main migration code will always be unaligned by virtue > of their small size. To workaround this issue, we'll require a second > file descriptor to be used exclusively for direct I/O. > > The second file descriptor can be obtained by QEMU by re-opening the > migration file (already possible), or by being provided by the user or > management application (support to be added in future patches). > > Signed-off-by: Fabiano Rosas <faro...@suse.de>
Reviewed-by: Peter Xu <pet...@redhat.com> -- Peter Xu
