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Re: quick help needed to calculate page sizes for logs

Posted By: Floyd Wellershaus
Date: Thursday, 10 February 2005, at 1:14 p.m.

In Response To: quick help needed to calculate page sizes for logs (Floyd Wellershaus )

This was very helpful, thank you.

Khaled Bentebal <khaled.bentebal@consult-ix.fr> wrote:
----- Original Message -----
From: "Floyd Welle...."
To:
Sent: Thursday, February 10, 2005 3:20 PM
Subject: quick help needed to calculate page sizes for logs [4230]


> Hi,
> We have a newly transaction logged database. We are going to do some
updates that will update all the rows in some tables inside a transaction.
So I am trying to ensure we have enough logs do do the transaction.
> I am approaching this from two points.
> 1) the logs.
> We have 12 logs at 12800 pages each, and a high water mark of 45%,
which tells me that we can only do a transaction of about 275 megs before a
long transaction occurs.
> No problem here.
>
> 2) I need to find how big the transactions will be on each of these
tables, and then, from what I understand, double that amount, as many
updates will need to store a before image and an after image of each row, in
the logs.
>
> Ok, so,
> for instance, oncheck -pt shows a table to have a maxrowsize of 1161, and
53,411 number of rows. I would think that if I multiply those 2 numbers and
divide by 4096 ( 4kpage size), I would get the approximate number of pages
needed to store that table.
> However, the number I get is 15,000 some odd pages, and according to the
oncheck -pt that table is only using around 5000 pages. Where is the
discrepancy, I must be doing somethign wrong here.
>

>>>>>>>>>>>>>>>> If your table occupies 5000 pages of 4K, then you must have
VARCHARS in your table. If your row is 1161 >>>>>>>>>>>>>>>> bytes long with
no VARCHARS, then you would store 3 rows at most per page (4096 - 28
overhead= 4068 >>>>>>>>>>>>>>>> usable; the 4 bytes are reserved per row in
the slot table); so you would need 17804 pages. So it obvious that
>>>>>>>>>>>>>>>> rows contain VARCHARS. I didn't take care of indexes that
could also be in the 5000 pages that you mention
>>>>>>>>>>>>>>>> since you didn't precise whether they are data pages, index
pages or both.
>>>>>>>>>>>>>>>> This is for the the number of pages that your table
occupies.

> Then I figure, if I can get the correct size of the table, multiply that
by 2, and just ensure that I add enough logs so that 45% of them will be
enough to store that amount of data.
>

>>>>>>>>>>>>>>>> As far as the log is concerned, the calculation is not that
easy but doable. Does your database use >>>>>>>>>>>>>>>> UNBUFFERED logging
or BUFFERED loging. This has a significant impact on the number of logs
used. In order >>>>>>>>>>>>>>>> to minimize the log space used, you could
either change to BUFFERED log for the database or use the following
>>>>>>>>>>>>>>>> SQL statement in your session : SET BUFFERED LOG;
>>>>>>>>>>>>>>>> Remember that each low level call in the log occupies some
bytes going from 24 to let us say 150 bytes at most. I
>>>>>>>>>>>>>>>> didn't calculate the exact size of the low levels calls
stored in the logs. Each instruction occupies a certain number
>>>>>>>>>>>>>>>> of bytes + the size of the row before and after the UPDATE
+ an instruction per index in that table. take a look at >>>>>>>>>>>>>>>>
you log using the onlog command which will give you an idea.
>>>>>>>>>>>>>>>> You can see that it is not easy to calculate teh size
precisely.
>>>>>>>>>>>>>>>> Other things that will have an impact on the log size
needed is the size of your transaction and the size of your >>>>>>>>>>>>>>>>
BUFSIZE (look in the onconfig file).
>>>>>>>>>>>>>>>> If you are updating all of the rows, can you drop your
indexes before you do this operation? This will save you >>>>>>>>>>>>>>>>
time ans log space.

I HOPE THAT HELPS YOU AND NOT CONFUSE YOU.

Khaled Bentebal
ConsultiX
Tél: 33 (0) 1 39 72 17 00
Fax: 33 (0) 1 39 72 17 01
Mobile: 33 (0) 6 07 78 41 97
Email: khaled.bentebal@consult-ix.fr
Site Web: http://www.consult-ix.fr


> Any advice on this would be appreciated.
>
> Thanks,
> floyd
>
>
> ========================
> -<>-
> Database Administrator
> Unix Administrator
>
> email: fwellers@yahoo.com
> Work: 703-733-4126
> Pager: 703-705-9241
> Email Pager: 7037059241@my2way.com
> Home: 703-430-0805
> Cell: 703-477-6045
> ========================
>
>
>
>




========================
-<<Floyd Wellershaus>>-
Database Administrator
Unix Administrator

email: fwellers@yahoo.com
Work: 703-733-4126
Pager: 703-705-9241
Email Pager: 7037059241@my2way.com
Home: 703-430-0805
Cell: 703-477-6045
========================


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