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 07:16 AM:
Why would the cache grow and grow? Since the delay, when it happens,
grows and grows, I get the feeling that this file system cache in RAM is
slowly getting bigger and bigger, and each time it needs to be flushed,
it takes longer and longer. If the cache is being emptied at some
reasonable point, why would it continue to grow? Remember that for each
mounted file system there is one process writing to a single file. The
disk usage remains 100% constant in terms of what is sent to be written.

Is there some policy or setting that controls how the file system deals
with file system cache in RAM? More specifically, is there any way to
limit it's size for a file system?

Is there a way to see how much of the RAM cache for a file system is
actually containing data waiting to be flushed?

Your problem is that you are treating the cache as a buffer.
Yes things get written, but the FS doesn't know you are running in 'write-only' mode so its retailing the most recently written and structural metadata just in case you want to read some if it back.

*THAT* is what a cache is about. Retaining stuff in case you want it again soon.

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.

Every now and again the caching algorithm need to 'flush' the 'least recently used' or sync metadata or somehow reorganise, or something, or something like that, depending on the implementation and algorithm.

As an earlier commentator pointed out, whiting to a raw disk avoids this :-)

Are there ways to control the cache? Probably, but it gets back to my point about you treating the cache as if it was a buffer.

You want buffering but not caching because, as far as I can see, you are doing 'write only'.

There are other factors such as 'commit times' and and matters to do with how the fie system reorgnaizes its b-trees, for example. All these can introduce the delays of which you speak.



--
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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