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Date: Tue, 09 Oct 2007 12:41:21 -0700
From: Darren Reed <darrenr@freebsd.org>
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To: Pawel Jakub Dawidek <pjd@FreeBSD.org>
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Cc: freebsd-fs@FreeBSD.org, freebsd-current@FreeBSD.org
Subject: Re: ZFS kmem_map too small.
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Pawel Jakub Dawidek wrote:
> Here are some updates:
>
> I was able to reproduce the panic by rsyncing big files and trying
> bonnie++ test suggested in this thread.
>
> Can you guys retry with this patch:
>
> 	http://people.freebsd.org/~pjd/patches/vm_kern.c.2.patch
>   

So, I have a question...
What happens if the "for (i = 0..)" is changed to "while(1)" and
the "panic" is subsequently removed?


It appears like the code changes the meaning of "WAIT" to "wait
for 4 seconds" then panic if it won't work.  Previously, "WAIT" was
not waiting at all...whch could be described as a bug!

If I recall correctly, ZFS caches writes and doe them in spurts and
that those spurts are spaced out more than 4 seconds.  (For the
curious, do "zpool status" and observe the gap in time between
write activity.)

If you start a large amount of I/O, it is possible that all the KVA will
be used up and ZFS will not get a chance to flush its buffers before
the 4s timer here expires.  Does that sound plausible?

Would doubling the 8 to (say) 16 be beneficial here, to at least make
the waiting span one ZFS flush out to disk?

Darren