maximum MAXBSIZE

[Gary Jennejohn]

On Thu, 9 Jan 2020 15:21:25 +0100 (CET)
Wojciech Puchar <wojtek at puchar.net> wrote:

> why FreeBSD default is so completely wrong for modern hardware?
> 
> i think 4MB is OK for HDDs, more may be optimal for RAID5 arrays.
> 

POLA (principle of least amazement).  I certainly don't need a MAXPHYS set
to 4MB on my desktop machine.  Not everyone using FreeBSD is running
servers with large amounts of memory and disk storage.

It's a trivial change if it's beneficial in a certain use scenario.  The
decision should be left up to the user.

> 
> On Thu, 9 Jan 2020, Warner Losh wrote:
> 
> > Netflix runs our entire network at MAXPHYS=8MB since we're doing huge reads off HDD.
> > Warner
> > 
> > 
> > On Thu, Jan 9, 2020 at 2:58 AM Wojciech Puchar <wojtek at puchar.net> wrote:
> >       2MB MAXPHYS was what i have set for over 3 years without problems.
> >
> >       On Wed, 8 Jan 2020, Conrad Meyer wrote:
> >  
> >       > Bufs are dynamically allocated from uma now, and perhaps a middle ground BSIZE is worth considering? Would 1MB and 2kB  
> >       bufs (1kB  
> >       > 32-bit) be awful?
> >       >
> >       > Cheers,
> >       > Conrad__
> >       >
> >       > On Wed, Jan 8, 2020 at 06:18 Konstantin Belousov <kostikbel at gmail.com> wrote:
> >       >__ __ __ __On Wed, Jan 08, 2020 at 02:52:57PM +0100, Wojciech Puchar wrote:
> >       >__ __ __ __> sorry i made a mistake - i change MAXPHYS not MAXBSIZE.
> >       >__ __ __ __>
> >       >__ __ __ __> 16MB works for now without problems
> >       >__ __ __ __MAXPHYS 16MB means that sizeof(struct buf) is around 32K (16K on 32bit).
> >       >
> >       >__ __ __ __>
> >       >__ __ __ __> On Wed, 8 Jan 2020, Gary Jennejohn wrote:
> >       >__ __ __ __>
> >       >__ __ __ __> > On Tue, 7 Jan 2020 22:47:54 +0000
> >       >__ __ __ __> > Rick Macklem <rmacklem at uoguelph.ca> wrote:
> >       >__ __ __ __> >
> >       >__ __ __ __> > > Hans Petter Selasky wrote:
> >       >__ __ __ __> > > > On 2020-01-07 22:12, Wojciech Puchar wrote:
> >       >__ __ __ __> > > > > default MAXBSIZE is 128kB. badly low for todays magnetic disks.
> >       >__ __ __ __> > > > >
> >       >__ __ __ __> > > > > i have it set to 2MB on all computers that have magnetic disks. Great
> >       >__ __ __ __> > > > > improvement with large files. especially when more than one are
> >       >__ __ __ __> > > > > read/wrote in parallel. And no problems experienced
> >       >__ __ __ __> > > > >
> >       >__ __ __ __> > > > > But for optimal performance MAXBSIZE should be transfered in few times
> >       >__ __ __ __> > > > > longer than average seek time. todays disk do 200-250MB/s so 2MB is
> >       >__ __ __ __> > > > > transfered below 10ms.
> >       >__ __ __ __> > > > >
> >       >__ __ __ __> > > > > 8-16MB seems like good choice. is there any reason not to set it that high?
> >       >__ __ __ __> > > >
> >       >__ __ __ __> > > > Old disk may not support it, especially USB 1.0/2.0 disks.
> >       >__ __ __ __> > > I also thought it was limited to MAXPHYS, but maybe I'm only thinking of the NFS
> >       >__ __ __ __> > > specific case?
> >       >__ __ __ __> > >
> >       >__ __ __ __> >
> >       >__ __ __ __> > There's a comment in param.h that it should not exceed MAXPHYS to be
> >       >__ __ __ __> > on the safe side.__ How old that comment is I can't say and that may
> >       >__ __ __ __> > not be the case today.
> >       >__ __ __ __> >
> >       >__ __ __ __> > MAXBSIZE is only 64KiB in my param.h.
> >       >__ __ __ __> >
> >       >__ __ __ __> > I have to agree with HPS.__ There are many old bridge-chips still in
> >       >__ __ __ __> > use and problems with a large MAXBSIZE might occur.__ It's certainly
> >       >__ __ __ __> > not uncommon to see capacity limitations - I have a docking station
> >       >__ __ __ __> > which can't see more than 3TB.
> >       >__ __ __ __> >
> >       >__ __ __ __> > --
> >       >__ __ __ __> > Gary Jennejohn
> >       >__ __ __ __> >
> >       >__ __ __ __> >
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> >       >  
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-- 
Gary Jennejohn