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To: Rick Macklem <rmacklem@uoguelph.ca>
Subject: Re: RFC: NFS client patch to reduce sychronous writes 
In-reply-to: <661293549.22251424.1385592648623.JavaMail.root@uoguelph.ca> 
Date: Wed, 27 Nov 2013 15:20:14 -0800
From: Kirk McKusick <mckusick@mckusick.com>
Cc: FreeBSD FS <freebsd-fs@freebsd.org>
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> Date: Wed, 27 Nov 2013 17:50:48 -0500 (EST)
> From: Rick Macklem <rmacklem@uoguelph.ca>
> To: Konstantin Belousov <kostikbel@gmail.com>
> Subject: Re: RFC: NFS client patch to reduce sychronous writes
> 
> Kostik wrote:
>> Sorry, I do not understand the question. mmap(2) itself does not change
>> file size.  But if mmaped area includes the last page, I still think
>> that the situation I described before is possible.
> 
> Yes, I'll need to look at this. If it is a problem, all I can think of
> is bzeroing all new pages when they're allocated to the buffer cache.
> 
> Thanks for looking at it, rick
> ps: Btw, jhb@'s patch didn't have the bzeroing in it.

The ``fix'' of bzero'ing every buffer cache page was made to UFS/FFS
for this problem and it killed write performance of the filesystem
by nearly half. We corrected this by only doing the bzero when the
file is mmap'ed which helped things considerably (since most files
being written are not also bmap'ed).

	Kirk