Re: [PATCH] 498+ days uptime

[Zlatko Calusic <Zlatko.Calusic@CARNet.hr>]

ebiederm@inetnebr.com (Eric W. Biederman) writes:

> >>>>> "BH" == Bernhard Heidegger <bheide@hyperwave.com> writes:
> 
> >>>>> ">" == 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.
> 
> BH> My version of update (something around Slakware 3.4) does the following:
> BH> 1.) calls bdflush(1,0) (fs/buffer.c:sys_bdflush) which will call
> BH>     sync_old_buffers() and return
> BH> 2.) only if the bdflush(1,0) fails (it returns < 0) it returns to the
> BH>     old behavior of sync()ing every 30 seconds
> 
> BH> But case 2) should only happen on really old kernels; on newer kernels
> BH> (I'm using 2.0.34) the bdflush() should never fail.
> 
> BH> But as I told, sync_old_buffers() do:
> BH> 1.) sync_supers(0)
> BH> 2.) sync_inodes(0)
> BH> 3.) go through dirty buffer list and may flush some buffers
> 
> BH> Conclusion: the meta data get synced every 5 seconds and some buffers may
> BH> be flushed.
> 
> >>> Questions is why can't this functionality be integrated in the kernel, 
> >>> so we don't have to run yet another daemon?
> 
> We can do this in kernel thread but I don't see the win.
> 

One daemon less to run.

This should be enough.

You have one less process running, you free some memory, and make
things slightly cleaner.

Not a big win, but small things make people happy. :)
-- 
Posted by Zlatko Calusic           E-mail: <Zlatko.Calusic@CARNet.hr>
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