It is essential to choose a reasonable high value for 'msize' to avoid severely degraded file I/O performance. This parameter can only be chosen on client/guest side, and a Linux client defaults to an 'msize' of only 8192 if the user did not explicitly specify a value for 'msize', which results in very poor file I/O performance.
Unfortunately many users are not aware that they should specify an appropriate value for 'msize' to avoid severe performance issues, so log a performance warning (with a QEMU wiki link explaining this issue in detail) on host side in that case to make it more clear. Currently a client cannot automatically pick a reasonable value for 'msize', because a good value for 'msize' depends on the file I/O potential of the underlying storage on host side, i.e. a feature invisible to the client, and even then a user would still need to trade off between performance profit and additional RAM costs, i.e. with growing 'msize' (RAM occupation), performance still increases, but performance delta will shrink continuously. Signed-off-by: Christian Schoenebeck <qemu_...@crudebyte.com> --- hw/9pfs/9p.c | 9 +++++++++ 1 file changed, 9 insertions(+) diff --git a/hw/9pfs/9p.c b/hw/9pfs/9p.c index 7bb994bbf2..99b6f24fd6 100644 --- a/hw/9pfs/9p.c +++ b/hw/9pfs/9p.c @@ -1353,6 +1353,15 @@ static void coroutine_fn v9fs_version(void *opaque) goto out; } + /* 8192 is the default msize of Linux clients */ + if (s->msize <= 8192) { + warn_report_once( + "9p: degraded performance: a reasonable high msize should be " + "chosen on client/guest side (chosen msize is <= 8192). See " + "https://wiki.qemu.org/Documentation/9psetup#msize for details." + ); + } + marshal: err = pdu_marshal(pdu, offset, "ds", s->msize, &version); if (err < 0) { -- 2.20.1