Re: [PATCH] 9pfs: log warning if msize <= 8192

[Daniel P . Berrangé]

On Wed, Sep 02, 2020 at 01:22:49PM +0200, Christian Schoenebeck wrote:
> It is essential to choose a reasonable high value for 'msize' to avoid
> severe degraded file I/O performance. This parameter has to 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'.
> 
> 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 on host side in that case to make it more
> clear.

What is a more reasonable "msize" value to pick instead of 8k ?
ie at what msize is I/O not several degraded ?

If there a reason that Linux can't pick a better default ?
 
> 
> Signed-off-by: Christian Schoenebeck <qemu_oss@crudebyte.com>
> ---
>  hw/9pfs/9p.c | 8 ++++++++
>  1 file changed, 8 insertions(+)
> 
> diff --git a/hw/9pfs/9p.c b/hw/9pfs/9p.c
> index 7bb994bbf2..33e948d636 100644
> --- a/hw/9pfs/9p.c
> +++ b/hw/9pfs/9p.c
> @@ -1353,6 +1353,14 @@ 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)."
> +        );
> +    }
> +
>  marshal:
>      err = pdu_marshal(pdu, offset, "ds", s->msize, &version);
>      if (err < 0) {
> -- 
> 2.20.1
> 
> 

Regards,
Daniel
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