Mailinglist Archive: opensuse (1239 mails)

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Re: [opensuse] Re: XFS and openSUSE 12.1
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.

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.

You might be able to set up a 'write-though' cache, that is one which still retains material but does write immediately. That will overcome the delays you describe.

The only way to no use the cache is to not mount a file system. As other people have said, write to the raw disk.



--
How long did the whining go on when KDE2 went on KDE3?

The only universal constant is change. If a species can not adapt it
goes extinct. That's the law of the universe, adapt or die.
-- Billie Walsh, May 18 2013
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