Comments on: Video and Slides: How InnoDB works

By: Kayra Otaner

Kayra Otaner — Fri, 20 Nov 2009 19:13:26 +0000

Sheeri,
Thank you, I believe we agree on that.
I just wanted to confirm there is no direct relationship between dirty pages & log flushes as that sentence I quoted sounded as if there is one.

Thank you again.

By: Sheeri

Sheeri — Fri, 20 Nov 2009 17:27:18 +0000

Kayra —

I’m not sure what the question is…you commented on me saying that dirty pages are not flushed to disk, even when logs are…you quoted me saying:

“modified db pages are pages not flushed to disk yet (>0 even when flush_logs_at_trx_commit=0).”

so I think we’re agreeing here….flushing logs has nothing to do with flushing dirty pages…..

Unless I missed something in your original question?

By: Kayra Otaner

Kayra Otaner — Thu, 19 Nov 2009 21:35:40 +0000

Sorry I should have been more clear with my question above.
My question was more like to understand relationship you came up with between dirty page count and innodb_flush_log_at_trx_commit variable.
AFAIK innodb_flush_log_at_trx_commit=1 guarantees 1 commit – 1 flush. Any other variation will (almost) provide flush every second. As you said, flushes are for logs only, so when you flush, you actually flush into log file, not to InnoDB table space. I believe flushing into log, whether its with every commit or every second should not reduce dirty pages in memory as they’re still not identical to the pages on InnoDB table space.

By: Sheeri

Sheeri — Thu, 19 Nov 2009 20:24:05 +0000

Kayra —

Yes, I can say that.

innodb_flush_log_at_trx_commit flushes logs to disk, not dirty pages. If the database crashes, InnoDB’s crash recovery will redo the writes on the dirty pages using what’s in the logs.

The flushes are for logs only, not the actual data pages. That tripped me up when I first thought about it too.

By: Kayra Otaner

Kayra Otaner — Thu, 19 Nov 2009 18:05:30 +0000

Can you really say :
“modified db pages are pages not flushed to disk yet (>0 even when flush_logs_at_trx_commit=0).”

Only innodb_flush_log_at_trx_commit=1 (default/Acid compliant) mode guarantees flushes with every commit. So anything other than default value 1, will flush logs to disk. But still this isn’t directly related to dirty pages?

By: Shlomi Noach

Shlomi Noach — Fri, 16 Oct 2009 19:31:31 +0000

Sheeri,

Haven’t watched it yet. Will find the time after the weekend :)

By: Sheeri Cabral

Sheeri Cabral — Fri, 16 Oct 2009 19:30:02 +0000

Shlomi — did you like the video too?

By: Sheeri Cabral

Sheeri Cabral — Fri, 16 Oct 2009 19:29:43 +0000

Shlomi — yep! Usually people just want the notes, so I use the trick of opening the first page to the first page of the notes. (mostly it bothered me when I printed, b/c when I print I want *only* the notes. So for a 10-slide deck, I print pages 11-20).

But I appreciate your trick!

By: Shlomi Noach

Shlomi Noach — Fri, 16 Oct 2009 19:17:02 +0000

Hi Sheeri,

Thanks for sharing the slides!
A note: I’m taking a wild guess, that you created the presentation with OpenOffice, then exported to PDF. I’m guessing so because of the nasty “feature” of OpenOffice to first generate slides-only pages, then slides+notes pages.

You may find my following howto useful: http://code.openark.org/blog/openoffice/how-to-export-openoffice-presentations-to-pdf

Regards