Re: [PATCH] 498+ days uptime

[Bernhard Heidegger <bheide@hyperwave.com>]

>>>>> ">" == Zlatko Calusic <Zlatko.Calusic@CARNet.hr> writes:

>> Bernhard Heidegger <bheide@hyperwave.com> writes:
>> >>>>> ">" == Zlatko Calusic <Zlatko.Calusic@CARNet.hr> writes:
>> 
>> >> "H. Peter Anvin" <hpa@transmeta.com> writes:
>> >> > 
>> >> > bdflush yes, but update is not obsolete.
>> >> > 
>> >> > It is still needed if you want to make sure data (and metadata)
>> >> > eventually gets written to disk.
>> >> > 
>> >> > Of course, you can run without update, but then don't bother if you
>> >> > lose file in system crash, even if you edited it and saved it few
>> >> > hours ago. :)
>> >> > 
>> >> > Update is very important if you have lots of RAM in your computer.
>> >> > 
>> >> 
>> >> Oh.  I guess my next question then is "why", as why can't this be done
>> >> by kflushd as well?
>> >> 
>> 
>> >> To tell you the truth, I'm not sure why, these days.
>> 
>> >> I thought it was done this way (update running in userspace) so to
>> >> have control how often buffers get flushed. But, I believe bdflush
>> >> program had this functionality, and it is long gone (as you correctly
>> >> noticed).
>> 
>> IMHO, update/bdflush (in user space) calls sys_bdflush regularly. This
>> function (fs/buffer.c) calls sync_old_buffers() which itself sync_supers
>> and sync_inodes before it goes through the dirty buffer lust (to write
>> some dirty buffers); the kflushd only writes some dirty buffers dependent
>> on the sysctl parameters.
>> If I'm wrong, please feel free to correct me!
>> 

>> You are not wrong.

>> Update flushes metadata blocks every 5 seconds, and data block every
>> 30 seconds.

My version of update (something around Slakware 3.4) does the following:
1.) calls bdflush(1,0) (fs/buffer.c:sys_bdflush) which will call
    sync_old_buffers() and return
2.) only if the bdflush(1,0) fails (it returns < 0) it returns to the
    old behavior of sync()ing every 30 seconds

But case 2) should only happen on really old kernels; on newer kernels
(I'm using 2.0.34) the bdflush() should never fail.

But as I told, sync_old_buffers() do:
1.) sync_supers(0)
2.) sync_inodes(0)
3.) go through dirty buffer list and may flush some buffers

Conclusion: the meta data get synced every 5 seconds and some buffers may
be flushed.

>> Questions is why can't this functionality be integrated in the kernel, 
>> so we don't have to run yet another daemon?

Good question, but I've another one: IMHO sync_old_buffers (especially
the for loop) do similar things as the kflushd. Why??
Is it possible to reduce the sync_old_buffers() routine to soemthing like:
{
  	sync_supers(0);
	sync_inodes(0);
}
??

Bernhard

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-----------------------------------------------------------------------------
Bernhard Heidegger                                       bheide@hyperwave.com
                  Hyperwave Software Research & Development
                       Schloegelgasse 9/1, A-8010 Graz
Voice: ++43/316/820918-25                             Fax: ++43/316/820918-99
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