Hello Fabiano, * First big thank you for starting/writing this document. It is a great resource.
On Fri, 7 Mar 2025 at 19:13, Fabiano Rosas <faro...@suse.de> wrote: > +++ b/docs/devel/migration/multifd.rst > @@ -0,0 +1,254 @@ > +Multifd > +Multifd is the name given for the migration capability that enables > +data transfer using multiple threads. Multifd supports all the > +transport types currently in use with migration (inet, unix, vsock, > +fd, file). * Multifd is Multiple File Descriptors, right? Ie. Does it work with one thread but multiple file descriptors? OR one thread per file descriptor is always the case? I have not used/tried 'multifd + file://' migration, but I imagined there one thread might be able to read/write to multiple file descriptors at a time. > +Usage > +----- > + > +On both source and destination, enable the ``multifd`` capability: > + > + ``migrate_set_capability multifd on`` > + > +Define a number of channels to use (default is 2, but 8 usually > +provides best performance). > + > + ``migrate_set_parameter multifd-channels 8`` > + * I get that this is a QEMU documentation, but for users/reader's convenience it'll help to point to libvirt:virsh migrate usage here -> https://www.libvirt.org/manpages/virsh.html#migrate , just as an alternative. Because doing migration via QMP commands is not as straightforward, I wonder who might do that and why. > +Restrictions > +------------ > + > +For migration to a file, support is conditional on the presence of the > +mapped-ram capability, see `mapped-ram`. > + > +Snapshots are currently not supported. * Maybe: Sanpshot using multiple threads (multifd) is not supported. > +`postcopy` migration is currently not supported. * Maybe - 'postcopy' migration using multiple threads (multifd) is not supported. ie. 'postcopy' uses a single thread to transfer migration data. * Reason for these suggestions: as a writer it is easy to think everything written in this page is to be taken with multifd context, but readers may not do that, they may take sentences in isolation. (just sharing thoughts) > +Multifd consists of: > + > +- A client that produces the data on the migration source side and > + consumes it on the destination. Currently the main client code is > + ram.c, which selects the RAM pages for migration; * So multifd mechanism can be used to transfer non-ram data as well? I thought it's only used for RAM migration. Are device/gpu states etc bits also transferred via multifd threads? > +- A packet which is the final result of all the data aggregation > + and/or transformation. The packet contains: a *header* with magic and > + version numbers and flags that inform of special processing needed > + on the destination; a *payload-specific header* with metadata referent > + to the packet's data portion, e.g. page counts; and a variable-size > + *data portion* which contains the actual opaque payload data. * It'll help to define the exact packet format here. Like they do in RFCs. Thank you for writing this. --- - Prasad
