Zero Out Deleted Pages - how does this work?   

Anyone know much about this?

I see from this page:

http://www.microsoft.com/technet/prodtechnol/exchange/guides/StoragePerformance/1c471676-2312-4ffe-adf2-15a9cfd529c4.mspx

"If you configure your server to zero out deleted database pages,
every time you delete an item from the database, multiple pages are
deleted. Exchange will then overwrite the deleted pages with zeros.
This causes much more disk I/O (potentially twice as much) than if you
did not enable this feature."

This is for Exchange 2003 and it sounds like its a I/O nightmare all
day long.  But then I read this article for Exchange 2000.

http://support.microsoft.com/default.aspx?scid=kb;%5BLN%5D;815068

"With Zero out deleted database pages selected, the data contained in
unused pages is overwritten with various characters (either 'z', 'd',
'l', or 'u', depending on the type of page being overwritten) during
an online backup. As each database page is written to the tape, the
page is then overwritten with zeros in the database on the hard disk.
After the backup is complete, the deleted data is on the tape but is
no longer in the database and cannot be recovered by using
conventional means."

This make it sound like its only done when the Online Backup runs, not
so much an I/O nightmare all day long - just during the backup.

Did they change something in Exchange 2003 that makes Zero Out work in
a realtime sense or is it still just when the backup runs?

Thanks,

Rob

date: Wed, 01 Feb 2006 11:20:28 -0500   author:   Rob Bergin

Re: Zero Out Deleted Pages - how does this work?   

I started to work with ESE in 2002 (the year), and I didn't remember
doing anything with database scrubbing.

So I just checked the code, and sure enough, there isn't a whole lot
of difference between scrubbing in 2000 and 2003 (Exchange versions,
not years).

I think what the first link is talking about is that during online
defrag there is another task that runs to scrub the old data. So that
additional IO is in fact done, but it should be only during the
nightly maintenance phase, not throughout the day.

Does that clear things up?

-martin

Boring-but-necessary-disclaimer: This posting is provided "AS IS" with
no warranties, and confers no rights.


In article <#nQCtt0JGHA.1312@TK2MSFTNGP09.phx.gbl>,
Rob Bergin   wrote:
>
>Anyone know much about this?
>
>I see from this page:
>
>http://www.microsoft.com/technet/prodtechnol/exchange/guides/StoragePerformance/1c471676-2312-4ffe-adf2-15a9cfd529c4.mspx
>
>"If you configure your server to zero out deleted database pages,
>every time you delete an item from the database, multiple pages are
>deleted. Exchange will then overwrite the deleted pages with zeros.
>This causes much more disk I/O (potentially twice as much) than if you
>did not enable this feature."
>
>This is for Exchange 2003 and it sounds like its a I/O nightmare all
>day long.  But then I read this article for Exchange 2000.
>
>http://support.microsoft.com/default.aspx?scid=kb;%5BLN%5D;815068
>
>"With Zero out deleted database pages selected, the data contained in
>unused pages is overwritten with various characters (either 'z', 'd',
>'l', or 'u', depending on the type of page being overwritten) during
>an online backup. As each database page is written to the tape, the
>page is then overwritten with zeros in the database on the hard disk.
>After the backup is complete, the deleted data is on the tape but is
>no longer in the database and cannot be recovered by using
>conventional means."
>
>This make it sound like its only done when the Online Backup runs, not
>so much an I/O nightmare all day long - just during the backup.
>
>Did they change something in Exchange 2003 that makes Zero Out work in
>a realtime sense or is it still just when the backup runs?
>
>Thanks,
>
>Rob

date: Thu, 16 Feb 2006 06:43:07 +0000 (UTC)   author:   Martin Chisholm [MSFT]