Buff Turnover Calc

Art, According to this post, Buff Turnover=(pagreads + rewrite)/BUFFERS Please clarify. Thanks <It means exactly what it says it means, towit that you are recycling <all of your buffers <every 4 to 10 minutes (or recycling a smaller number even more <frequently which is <unlikely given the LRU replacement algorithm). It means that 6-14 <times during each hour (aggregating both of your results) applications <are looking to read pages <that would have already been in cache if the cache were larger but <which were likely <swapped out by more recently, but perhaps less frequently, used <pages. You decide how much a larger cache could have sped up those <millions of page accesses! Now temper this by applying your writ <cache ratio to bufwrits to get a <lower bound for turnovers, the figure I calculate is an upper bound I <use to detect <potential problems. Using the figures Bob provides below I calculate, <for the single <15 hour period: <Upper bound BT = (2336746 + 8962516) / 126000 = 89.67668 <Upper bound BTR = 89.68 / 15 = 5.98 Turnovers per hour <Lower bound BT = ((2336746 * (1.0-0.9481) + 8962516) / 126000 = <72.09359 Lower bound BTR = 72.09 / 15 = 4.81 Turnovers per hour <You can see that even a 94+% write cache percentage has little effect <on the <turnover calculation which is why I normally just use the upper bound <calculation. <These figures are probably not bad but there is ALWAYS room for <improvement. <To Bob's last question below, yes you might reduce the SHMVIRTSIZE and trade <it for more buffers. Also your bufwaits ratio is around 5% in the figures shown here <but if at other times you are seeing figures between 8 and 10% then <try more LRUs <and the appropriate number of CLEANERS to improve things, note that <increasing <BUFFERS will tend to help the BR also if the LRU contention is coming <mainly <during peak load for short periods so try that first. <Art S. Kagel Robert Bothwell wrote: > Chris Burton wrote: > > > >Buffer Turnovers = (pagreads + bufwrits) / BUFFERS > > > > What is a 'few' time an hour. I am showing 11 times per hour with a > > buf_wait ratio of about 5%. Comments? > > > [snip] > > Chris > > I see similar numbers, though my buf_wait ratio regularly visits no- mans > land > (8-10). We're running 7.31.UC5 on a 4-way IBM H50 (332 MHZ) w/2 GB RAM, > & AIX > 4.3.2. I am seeing a turnover rate anywhere from 6 to 14 per hour. This > is 'onstat -p' > from our system running about 15 hours since the last onstat -z: > > Informix Dynamic Server Version 7.31.UC5 -- On-Line -- Up 2 days > 09:29:11 -- > 915312 Kbytes > > Profile > dskreads pagreads bufreads %cached dskwrits pagwrits bufwrits %cached > 7048290 8962516 472042006 98.51 605618 1379970 11671757 94.81 > > isamtot open start read write rewrite delete commit > rollbk > 408326705 4814943 29625295 269744131 4571378 2336746 87967 > 111289 210 > > gp_read gp_write gp_rewrt gp_del gp_alloc gp_free gp_curs > 0 0 0 0 0 0 0 > > ovlock ovuserthread ovbuff usercpu syscpu numckpts flushes > 0 0 0 24235.85 11054.95 85 196 > > bufwaits lokwaits lockreqs deadlks dltouts ckpwaits compress seqscans > 1339052 91 157160383 0 0 304 35176 2096917 > > ixda-RA idx-RA da-RA RA-pgsused lchwaits > 1435270 95970 2852979 4376467 49452 > > Here is onstat -g seg from mid-day: > > Segment Summary: > id key addr size ovhd class blkused blkfree > 1 1381451777 30000000 534626304 9016 R 65256 6 > 2 1381451778 50000000 402653184 6740 V 24576 24576 > Total: - - 937279488 - - 89832 24582 > > (* segment locked in memory) > > From config file: > > BUFFERS 126000 # Maximum number of shared buffers > SHMVIRTSIZE 393216 # initial virtual shared memory segment> size > > I suppose we could reduce SHMVIRTSIZE (since we don't use what we > allocate) and > increase the BUFFERS to see if we can reduce the BTR? > > drbob Sent via Deja.com http://www.deja.com/ Before you buy.

cyclonejls@my-deja.com wrote: > > Art, According to this post, Buff Turnover=(pagreads + rewrite)/BUFFERS BTR (Upper) = ((pagreads + bufwrites) / buffers) / time period I generaly use hours since startup or last onstat -z whichever is more recent. looking for a small rate say <6 times per hour. Also look at BTF which is the inverse or hours/minutes/secs between turnovers. How much more elaborate do you want? Art S. Kagel > Please clarify. > > Thanks > > <It means exactly what it says it means, towit that you are recycling > <all of your buffers > <every 4 to 10 minutes (or recycling a smaller number even more > <frequently which is > <unlikely given the LRU replacement algorithm). It means that 6-14 > <times during each hour (aggregating both of your results) applications > <are looking to read pages > <that would have already been in cache if the cache were larger but > <which were likely > <swapped out by more recently, but perhaps less frequently, used > <pages. You decide how much a larger cache could have sped up those > <millions of page accesses! Now temper this by applying your writ > <cache ratio to bufwrits to get a > <lower bound for turnovers, the figure I calculate is an upper bound I > <use to detect > <potential problems. Using the figures Bob provides below I calculate, > <for the single > <15 hour period: > > <Upper bound BT = (2336746 + 8962516) / 126000 = 89.67668 > <Upper bound BTR = 89.68 / 15 = 5.98 Turnovers per hour > > <Lower bound BT = ((2336746 * (1.0-0.9481) + 8962516) / 126000 = > <72.09359 Lower bound BTR = 72.09 / 15 = 4.81 Turnovers per hour > > <You can see that even a 94+% write cache percentage has little effect > <on the > > <turnover calculation which is why I normally just use the upper bound > <calculation. > <These figures are probably not bad but there is ALWAYS room for > <improvement. > > <To Bob's last question below, yes you might reduce the SHMVIRTSIZE and > trade > > <it for more buffers. Also your bufwaits ratio is around 5% in the > figures shown here > <but if at other times you are seeing figures between 8 and 10% then > <try more LRUs > <and the appropriate number of CLEANERS to improve things, note that > <increasing > <BUFFERS will tend to help the BR also if the LRU contention is coming > <mainly > > <during peak load for short periods so try that first. > > <Art S. Kagel > > Robert Bothwell wrote: > > > Chris Burton wrote: > > > > > >Buffer Turnovers = (pagreads + bufwrits) / BUFFERS > > > > > > What is a 'few' time an hour. I am showing 11 times per hour with a > > > buf_wait ratio of about 5%. Comments? > > > > > [snip] > > > Chris > > > > I see similar numbers, though my buf_wait ratio regularly visits no- > mans > > land > > (8-10). We're running 7.31.UC5 on a 4-way IBM H50 (332 MHZ) w/2 GB > RAM, > > & AIX > > 4.3.2. I am seeing a turnover rate anywhere from 6 to 14 per hour. > This > > is 'onstat -p' > > from our system running about 15 hours since the last onstat -z: > > > > Informix Dynamic Server Version 7.31.UC5 -- On-Line -- Up 2 days > > 09:29:11 -- > > 915312 Kbytes > > > > Profile > > dskreads pagreads bufreads %cached dskwrits pagwrits bufwrits %cached > > 7048290 8962516 472042006 98.51 605618 1379970 11671757 94.81 > > > > isamtot open start read write rewrite delete commit > > rollbk > > 408326705 4814943 29625295 269744131 4571378 2336746 87967 > > 111289 210 > > > > gp_read gp_write gp_rewrt gp_del gp_alloc gp_free gp_curs > > 0 0 0 0 0 0 0 > > > > ovlock ovuserthread ovbuff usercpu syscpu numckpts flushes > > 0 0 0 24235.85 11054.95 85 196 > > > > bufwaits lokwaits lockreqs deadlks dltouts ckpwaits compress > seqscans > > 1339052 91 157160383 0 0 304 35176 2096917 > > > > ixda-RA idx-RA da-RA RA-pgsused lchwaits > > 1435270 95970 2852979 4376467 49452 > > > > Here is onstat -g seg from mid-day: > > > > Segment Summary: > > id key addr size ovhd class blkused blkfree > > 1 1381451777 30000000 534626304 9016 R 65256 6 > > 2 1381451778 50000000 402653184 6740 V 24576 24576 > > Total: - - 937279488 - - 89832 24582 > > > > (* segment locked in memory) > > > > From config file: > > > > BUFFERS 126000 # Maximum number of shared buffers > > SHMVIRTSIZE 393216 # initial virtual shared memory segment> > size > > > > I suppose we could reduce SHMVIRTSIZE (since we don't use what we > > allocate) and > > increase the BUFFERS to see if we can reduce the BTR? > > > > drbob > > Sent via Deja.com http://www.deja.com/ > Before you buy.

Is "Bufwrites" labeled "rewrite" from the onstat -p output? The value used in the calculation below is labeled "rewrite". Relevant info below. Initially, I thought it was "bufwrits". From reading other posts, I do believe others have made the same mistake. Thanks. In article <39BD4538.BFB6389@bloomberg.net>, kagel@bloomberg.net wrote: > cyclonejls@my-deja.com wrote: > > > > Art, According to this post, Buff Turnover=(pagreads + rewrite)/BUFFERS > > BTR (Upper) = ((pagreads + bufwrites) / buffers) / time period > > I generaly use hours since startup or last onstat -z whichever is more > recent. looking for a small rate say <6 times per hour. Also look at > BTF which is the inverse or hours/minutes/secs between turnovers. > > How much more elaborate do you want? > > Art S. Kagel > > > Please clarify. > > > > Thanks > > [SNIP] > > <Upper bound BT = (2336746 + 8962516) / 126000 = 89.67668 > > <Upper bound BTR = 89.68 / 15 = 5.98 Turnovers per hour > > > > <Lower bound BT = ((2336746 * (1.0-0.9481) + 8962516) / 126000 = > > <72.09359 Lower bound BTR = 72.09 / 15 = 4.81 Turnovers per hour > > [SNIP] > > > Informix Dynamic Server Version 7.31.UC5 -- On-Line -- Up 2 days > > > 09:29:11 -- > > > 915312 Kbytes > > > > > > Profile > > > dskreads pagreads bufreads %cached dskwrits pagwrits bufwrits % cached > > > 7048290 8962516 472042006 98.51 605618 1379970 11671757 94.81 > > > > > > isamtot open start read write rewrite delete commit > > > rollbk > > > 408326705 4814943 29625295 269744131 4571378 2336746 87967 > > > 111289 210 > > > > > > gp_read gp_write gp_rewrt gp_del gp_alloc gp_free gp_curs > > > 0 0 0 0 0 0 0 > > > > > > ovlock ovuserthread ovbuff usercpu syscpu numckpts flushes > > > 0 0 0 24235.85 11054.95 85 196 > > > > > > bufwaits lokwaits lockreqs deadlks dltouts ckpwaits compress > > seqscans > > > 1339052 91 157160383 0 0 304 35176 2096917 > > > > > > ixda-RA idx-RA da-RA RA-pgsused lchwaits > > > 1435270 95970 2852979 4376467 49452 [SNIP] Sent via Deja.com http://www.deja.com/ Before you buy.

cyclonejls@my-deja.com wrote: > > Is "Bufwrites" labeled "rewrite" from the onstat -p output? The value > used in the calculation below is labeled "rewrite". Relevant info > below. Initially, I thought it was "bufwrits". From reading other > posts, I do believe others have made the same mistake. You and other are not mistaken I was: It is indeed bufwrits from the first line of onstat -p (ie between pagwrits and %cached) not rewrite. You are confused because the numbers in my calculations below were incorrectly pulled from the output. I must have had my eyes crossed that day ;0) I did not realize the error until just now. Of course using the correct number just makes the situation look a bit worse since then the BT is 163 and BTR is 10.2 upper and the lowers are 75.93 and 5.06. Thanks for being bulldog on this until I wised up. Art S. Kagel > > Thanks. > > In article <39BD4538.BFB6389@bloomberg.net>, > kagel@bloomberg.net wrote: > > cyclonejls@my-deja.com wrote: > > > > > > Art, According to this post, Buff Turnover=(pagreads + > rewrite)/BUFFERS > > > > BTR (Upper) = ((pagreads + bufwrites) / buffers) / time period > > > > I generaly use hours since startup or last onstat -z whichever is > more > > recent. looking for a small rate say <6 times per hour. Also look at > > BTF which is the inverse or hours/minutes/secs between turnovers. > > > > How much more elaborate do you want? > > > > Art S. Kagel > > > > > Please clarify. > > > > > > Thanks > > > > [SNIP] > > > <Upper bound BT = (2336746 + 8962516) / 126000 = 89.67668 > > > <Upper bound BTR = 89.68 / 15 = 5.98 Turnovers per hour > > > > > > <Lower bound BT = ((2336746 * (1.0-0.9481) + 8962516) / 126000 = > > > <72.09359 Lower bound BTR = 72.09 / 15 = 4.81 Turnovers per hour > > > > [SNIP] > > > > Informix Dynamic Server Version 7.31.UC5 -- On-Line -- Up 2 > days > > > > 09:29:11 -- > > > > 915312 Kbytes > > > > > > > > Profile > > > > dskreads pagreads bufreads %cached dskwrits pagwrits bufwrits % > cached > > > > 7048290 8962516 472042006 98.51 605618 1379970 11671757 94.81 > > > > > > > > isamtot open start read write rewrite delete commit > > > > rollbk > > > > 408326705 4814943 29625295 269744131 4571378 2336746 87967 > > > > 111289 210 > > > > > > > > gp_read gp_write gp_rewrt gp_del gp_alloc gp_free gp_curs > > > > 0 0 0 0 0 0 0 > > > > > > > > ovlock ovuserthread ovbuff usercpu syscpu numckpts flushes > > > > 0 0 0 24235.85 11054.95 85 196 > > > > > > > > bufwaits lokwaits lockreqs deadlks dltouts ckpwaits compress > > > seqscans > > > > 1339052 91 157160383 0 0 304 35176 > 2096917 > > > > > > > > ixda-RA idx-RA da-RA RA-pgsused lchwaits > > > > 1435270 95970 2852979 4376467 49452 > [SNIP] > > Sent via Deja.com http://www.deja.com/ > Before you buy.