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/ -- //www.freelists.org/webpage/oracle-l