Mailinglist Archive: opensuse (1239 mails)

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[opensuse] Re: XFS and openSUSE 12.1 [SOLVEDish]
Carlos E. R. wrote:
I don't think it is that. It could be that the filesystem metadata is
not written, perhaps the journal. It makes no sense not to write all
the data as it gets it.

Maybe the metadata is not written till, say, 5 seconds of no activity,
or till it can not be delayed longer.
The write the journal whenever it gets something to write --
XFS journal only journals metadata -- and they can't change
the meta data on disk until the journal has sync w/the disk or
they risk fs corruption.

A data stream to disk -- one continuous stream waits
^^ that long (on my system, it's 3000 of 'em, so 30 seconds.
The file system should be synced every 30 seconds as well from:
in the same dir.

* the cache does not grow endlessly to full RAM
If you don't want to use the cache then don't.
It is doing what you told it to do -- you could write O_DIRECT
and the cache wouldn't grow due to your program at all.

But if you keep writing to the cache, it will mark those buffers as
containing **COPIES** of what is on disk -- so if something asks
for them it can return the info from memory.

But 30 seconds is the longest it should normally wait to write out

It can be set higher -- if you are on a laptop, 60-120 seconds
isn't bad... be sure to have a full charge on your battery though.

But this is what the Linux cache does by design :-)

* the flush, when it happens, does not take too long.
It takes -- he said less than 120ms/call.

It's not flushing to disk every half hour.

Normal Linux is not a Real Time Operating System by default, unless
you change some options in the kernel and add some utilities. Maybe
what you need is that.
It sounded like he was on a not too fast computer that might bog down
if it had to scan 10G all at once -- might even take 5 seconds!

A suggestion on the XFS list is that I may need to tweak some
variables, as suggested by Lars Müller and others earlier in the
thread. I just have not figured them out to know what to change. I
am working on that...
Oi...It's NOT going to be changed by tweaking XFS
vars. XFS isn't what is holding it in memory. It's the kernel.

Linux is saving up scanning for more memory for all at once, as it
is more efficient that way.

You can play with values in /proc/sys -- but it will all be a kludge,
as what you really need is to avoid the cache altogether.

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