Mailinglist Archive: opensuse (1239 mails)

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Re: [opensuse] Re: XFS and openSUSE 12.1
  • From: Roger Oberholtzer <roger@xxxxxx>
  • Date: Wed, 12 Jun 2013 16:09:59 +0200
  • Message-id: <1371046199.16366.46.camel@acme.pacific>
On Wed, 2013-06-12 at 06:50 -0400, Anton Aylward wrote:
Roger Oberholtzer said the following on 06/11/2013 08:15 AM:
That you don't, that you are treating the cacche as a buffer and its
behaving as a cache, is the root of that you are observing and
complaining about.
I am not doing this. I am only reporting that it seems that as the file
system gets written to, the OS cache grows and grows. As the cache
grows, the occasional flushes that are done to the disk seem to take
longer and longer. Coincidence? Perhaps. Repeatable? 100%.

What you are describing is the behaviour of a cache.

I don't know why you say "I am not doing this".
Perhaps you are not actively saying "I have programmed a cache", but the
file system is running a cache anyway because that is what the file
systems do. It not you, its the system you are running. It applies
just as much to XFS as ext4 as any other FS.

I understand that the kernel is doing this as the result of me making a
big file. So of course, ultimately, I am making it happen. What I am
trying to say is that I am not telling the kernel to act stupid when
making this big file so that all my memory goes to cache and that huge
delays are incurred when dealing with the cache.


But what you *are* doing when you flush with sync etc etc that you
described in previous email is _expecting_ the system cache to behave
like a buffer. When you flush a buffer it empties. When you 'flush' a
cache all you are doing is making sure that the disk is in sync with the
cache. The cache does not empty.

When you mount a FS the cache is empty, but every time you write or read
from the FS it sticks in the cache up to the maximum capacity of the
cache. The cache algorithm (probably) supposes that the most recently
accessed stuff is going to be re-used so only drops "old" stuff.

"maximum capacity of the cache" - there is the core of the problem. It
seems that the kernel thinks the max capacity is all my RAM! When I
limit the cache to at most 4 or so GB (the result of my fix), the system
acts as I expect. When I let the system alone to deal with the cache, it
grows until the system has a problem.

I am happy to let the system cache things as it usually does. But I am
unable to let it cache things as it seems to want to when a big file is
written at a steady rate and there is lots of available RAM for the
taking. Something in the logic seems to break. I just do not know what.

I am guessing some of the tunable parameters for the kernel VM may help.



Yours sincerely,

Roger Oberholtzer

Ramböll RST / Systems

Office: Int +46 10-615 60 20
Mobile: Int +46 70-815 1696
roger.oberholtzer@xxxxxxxxxx
________________________________________

Ramböll Sverige AB
Krukmakargatan 21
P.O. Box 17009
SE-104 62 Stockholm, Sweden
www.rambollrst.se


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