Re: one hour checkpoints

Amonte,

Yes you are right, dbwr does write in batches, whenever possible. So
this logic does not apply all the time. It was a maximum number of
possible writes.

I think Tanel's explalanation is more accurate with what's happening
in your case.

What I was trying to say is that there is no direct relationship
between size of redo and amount of time it takes to checkpoint those
changes. So sometimes it may take 2 hours to checkpoint 600mb of redo,
other times it may take 6 seconds, all depending on how many block
changed, and how many ios dbwriter is doing. Note: async IO also plays
a big role here. If you have it on, then dbwr will submit a lot of ios
to the OS. If you have it off (or not working) it will be 1 io, wait
for finish, next io, wait for finish. Of course 1 io could be more
then 1 block.

Where did you check the block size?

--
Christo Kutrovsky
Senior Database/System Administrator
The Pythian Group - www.pythian.com
I blog at http://www.pythian.com/blogs/

On 1/25/07, amonte <ax.mount@xxxxxxxxx> wrote:
Christo

Are you saying checkpoints write a block per I/O? I thought DBWR write
blocks in batches and not single blocks. Your theory doesnt sound right to
me. I have another database which generates 24GB redo per hour, if I use
your calculation 24GB will be 50331648 blocks then my I/O time would be

1000/(50331648/3600) = 0.0715 ms

Isnt that a bit too fast?

TIA

Alex




On 1/24/07, Christo Kutrovsky < kutrovsky.oracle@xxxxxxxxx> wrote:
> Well, in theory, 600mb of redo log could be 1'228'800 blocks changed.
> That's 629145600 bytes of redo log, with a redo record been 512 bytes.
>
> That's only an approximation, since you could have merged records,
> multiple changes in a single record and etc.
>
> Let's assume a typical IO time of 5 ms. That would be 1228800*5ms =
> 6'144'000 ms = 6'144 seconds = 1.7 hours.
>
> So it could've taken 1.7 hours. And that's been optimistic about the
> 5ms IO. It could've been 10 ms during busy times.
>
> It's not as bad as it sounds.
>
> --
> Christo Kutrovsky
> Senior Database/System Administrator
> The Pythian Group - www.pythian.com
> I blog at http://www.pythian.com/blogs/
>
>
> On 1/18/07, amonte <ax.mount@xxxxxxxxx> wrote:
> > Hello all
> >
> > I have a quite busy Oracle APPS database running in HPUX. Today during
peak
> > hour I saw this in alert
> >
> > Thu Jan 18 10:19:50 2007
> > Beginning log switch checkpoint up to RBA [0x17d1.2.10], SCN:
> > 0x0784.b14a52c1
> > Thu Jan 18 10:22:09 2007
> > Beginning log switch checkpoint up to RBA [ 0x17d2.2.10], SCN:
> > 0x0784.b14a8673
> > Thu Jan 18 10:26:18 2007
> > Beginning log switch checkpoint up to RBA [0x17d3.2.10], SCN:
> > 0x0784.b14aee22
> > Thu Jan 18 10:51:32 2007
> > Beginning log switch checkpoint up to RBA [ 0x17d4.2.10], SCN:
> > 0x0784.b14d0ddb
> > Thu Jan 18 11:09:24 2007
> > Beginning log switch checkpoint up to RBA [0x17d5.2.10], SCN:
> > 0x0784.b14ea5dd
> > .........................
> > Thu Jan 18 11:12:04 2007
> > Completed checkpoint up to RBA [0x17d1.2.10], SCN: 0x0784.b14a52c1
> > Thu Jan 18 11:16:19 2007
> > Completed checkpoint up to RBA [0x17d2.2.10], SCN: 0x0784.b14a8673
> > Thu Jan 18 11:16:45 2007
> > Completed checkpoint up to RBA [ 0x17d3.2.10], SCN: 0x0784.b14aee22
> > Thu Jan 18 11:18:22 2007
> > Completed checkpoint up to RBA [0x17d4.2.10], SCN: 0x0784.b14d0ddb
> > Thu Jan 18 11:23:56 2007
> > Completed checkpoint up to RBA [ 0x17d5.2.10 ], SCN: 0x0784.b14ea5dd
> >
> >
> >
> > From v$log_histroty I see the log switch frequency
> >
> > select * from v$log_history
> > where FIRST_TIME > ('20070118 090000');
> >
> >      RECID      STAMP    THREAD#  SEQUENCE#
 FIRST_CHANGE#
> > FIRST_TIME          NEXT_CHANGE#
> > ---------- ---------- ---------- ---------- ----------------------
> > --------------- ----------------
> >       6096  612181190          1       6096
8266491016370 20070118
> > 090018    8266491515585
> >       6097  612181328          1       6097
8266491515585 20070118
> > 101949    8266491528819
> >       6098  612181578          1       6098
8266491528819 20070118
> > 102208    8266491555362
> >       6099  612183092          1       6099
8266491555362 20070118
> > 102618    8266491694555
> >       6100  612184164          1       6100
8266491694555 20070118
> > 105132    8266491799005
> >       6101  612185404          1       6101
8266491799005 20070118
> > 110923    8266491936643
> >
> >
> > Redo log are 600MB each.
> >
> > How can a checkpoint take so long?
> >
> >
> >
> > Cheers
> >
> > Alex
> >
> >
>
>
> --
> Christo Kutrovsky
> Senior Database/System Administrator
> The Pythian Group - www.pythian.com
> I blog at http://www.pythian.com/blogs/
>




--
Christo Kutrovsky
Senior Database/System Administrator
The Pythian Group - www.pythian.com
I blog at http://www.pythian.com/blogs/
--
http://www.freelists.org/webpage/oracle-l


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