Heh... that's funny. That last message stopped at a lone period on the line.
:o) Here's the actual message.
Hi Neil,
I have a question regarding your bdflush settings. The kernel docs say the
following:
Table 2-2: Parameters in /proc/sys/vm/bdflush
..............................................................................
Value Meaning
nfract Percentage of buffer cache dirty to activate bdflush
ndirty Maximum number of dirty blocks to write out per wake-cycle
nrefill Number of clean buffers to try to obtain each time we call refill
nref_dirt buffer threshold for activating bdflush when trying to refill
buffers.
dummy Unused
age_buffer Time for normal buffer to age before we flush it
age_super Time for superblock to age before we flush it
dummy Unused
dummy Unused
..............................................................................
"The 5th and 6th numbers are significant."
-Is that really the case? Above, the 5th isn't even used.
age_buffer and age_super
------------------------
Finally, the age_buffer and age_super parameters govern the maximum time Linux
waits before writing out a dirty buffer to disk. The value is expressed in
jiffies (clockticks), the number of jiffies per second is 100. Age_buffer is
the maximum age for data blocks, while age_super is for filesystems meta data.
Could you explain how the "every 5 seconds it flushs data older than 30
seconds" works? I just don't get that from what the docs say. I'm just
somewhat confused now.
TIA,
-Mark
On Friday 02 May 2003 12:55 am, Neil Brown wrote:
> On Friday May 2, cchan@outblaze.com wrote:
> > Hi all,
> >
> > We are observing a consistent interval of about 4 minutes at which there
> > are large sustained writes to disk that causes mysqld to block and not
> > respond for the entire period.
> >
> > We are using data=journal with a 128M journal and the filesystem is
> > 150GB in size.
>
> Yep. This is due to some lazy code in ext3.
> If the journal fills up, then it flushes the whole journal before
> continuing.
>
> There are three ways to get around this problem.
> 1/ fix the code :-)
> 2/ use a bigger journal. There is a drawback with bigger journals
> though as replay after a crash will take longer.
> 3/ Push the bdflush parameters down so that data in the journal will
> be flushed out more often and the journal will not get full.
>
> Something like:
> echo 40 0 0 0 60 300 60 0 0 > /proc/sys/vm/bdflush
>
> The 5th and 6th numbers are significant.
> The defaults are 500 and 3000 (hundreths of a second) so
> every 5 seconds it flushs data older than 30 seconds. If your
> journal cannot hold 30 seconds of data, this is a problem.
> So drop the 3000 to maybe 300 or 500 (3 or 5 seconds) and then
> drop the 500 to a reasonable fraction of that (50 or so (half a
> second)).
>
> 2 and 3 are complimentary. The bigger the journal, the larger the
> age of flushed buffers can be.
>
> NeilBrown
>
>
>
> _______________________________________________
>
> Ext3-users@redhat.com
> https://listman.redhat.com/mailman/listinfo/ext3-users
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