Re: Re: ZFS file caching question

Kris Kennaway wrote:
> Andrew Gallatin wrote:
>> If I have, say, 512MB RAM and a 1GB file which is written or read
>> sequentially on an otherwise idle system, I'd expect the last 512MB (-
>> epsilon) of the file's pages to be cached in RAM.  This is true for
>> UFS, but it does not appear to be the case with ZFS (see example
>> below).
>>
>> Can somebody explain how the arc cache in ZFS relates to the normal
>> page cache used by traditional filesystems?  Are ZFS filesystems
>> cached exclusively in the arc cache, and not in the page cache?  Is
>> the arc cache per-filesystem, per-pool, or global for ZFS as a whole?
> 
> The ZFS arc cache is completely independent from the normal buffer cache 
> on FreeBSD.  This is inefficient in a number of ways.  I have also seen 
> things that make me suspicious that it is not caching properly even when 
> you tune it to be "large enough" (if possible given memory constraints), 
> but I haven't confirmed this.
> 
>> Hmm.. Could this be the cause of the problems with ZFS and mmap'ed files?
> 
> What problems do you mean?  There were coherency problems but I think 
> they were fixed.
> 
> Kris

I noticed in the release notes for 8.0-CURRENT under "What's New":

   http://www.freebsd.org/relnotes/CURRENT/relnotes/new.html

it says:

  "A problem with using mmap(2) on ZFS filesystems has been fixed."

Can anyone say which commits fixed this, and if they have been or will 
be MFCed to 7-STABLE?

I'm curious because I've been having lots of trouble with Cyrus IMAP on 
7.0-RELEASE corrupting cyrus.cache files, and was somewhat suspicious it 
could be mmap related.

	Barry