MAXBSIZE increase

[Konstantin Belousov]

On Fri, Mar 27, 2015 at 10:57:05PM +0200, Alexander Motin wrote:
> Hi.
> 
> Experimenting with NFS and ZFS I found an inter-operation issue: ZFS by
> default uses block of 128KB, while FreeBSD NFS (both client and server)
> is limited to 64KB requests by the value of MAXBSIZE. On file rewrite
> that limitation makes ZFS to do slow read-modify-write cycles for every
> write operation, instead of just writing the new data.  Trivial iozone
> test show major difference between initial write and rewrite speeds
> because of this issue.
> 
> Looking through the sources I've found and in r280347 fixed number of
> improper MAXBSIZE use cases in device drivers. After that I see no any
> reason why MAXBSIZE can not be increased to at least 128KB to match ZFS
> default (ZFS now supports block up to 1MB, but that is not default and
> so far rare). I've made a test build and also successfully created UFS
> file system with 128KB block -- not sure it is needed, but seems it
> survives this change well too.
> 
> Is there anything I am missing, or it is safe to rise this limit now?

This post is useless after the Bruce explanation, but I still want to 
highlidht the most important point from that long story:

increasing MAXBSIZE without tuning other buffer cache parameters
would dis-balance the buffer cache.  Allowing bigger buffers increases
fragmentation, while limiting the total number of buffers.  Also, it
changes the tuning for runtime limits for amount of io in flight, see
hi/lo runningspace initialization.