Re: Falconstor/IPStor with Clariion, EMC Timefinder with Symmetrix

From: Michael Boligan <michael.boligan.b@xxxxxxxxx>
To: oracle-l@xxxxxxxxxxxxx
Date: Wed, 24 Mar 2004 11:13:25 -0500



My explanation only applies to Compaq's Virtual Snapshot replicator software, I
wouldn't be surprised if NetApp had something better.

And besides, I have never known a salesperson to lie!!  : - o




|---------+----------------------------->
|         |           Tim Johnston      |
|         |           <tjohnston@quallab|
|         |           y.com>            |
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|         |           eelists.org       |
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|         |           03/24/2004 10:19  |
|         |           AM                |
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  |       Subject:  Re: Falconstor/IPStor with Clariion, EMC Timefinder with 
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>The way it works is that when you create a snapshot it creates a file which
contains a pointer to the database file.  When someone changes the database, the
before image is written to the snapshot file.  At this point the file is no
longer just a pointer it containts before image data.
>

Yuck...  I remember getting a dog and pony from the NetApps guys about
the snapshot method...  The way I understood their method was much
different then this...

A change to a physical block is always written to a new location in the
filesystem
There is a pointer for each physical block (call this the current
pointer list)
When a physical block is updated, it is written to a new block and
current pointer list is updated
You perform a snapshot copy which grabs a copy of these pointers (call
this the snapshot pointer list)
As long as the snapshot pointer list is active, those blocks can not be
changed or overwritten
This allows the two pointer lists to stay "mostly" the same
The longer a snapshot is active, the more the two pointer lists will diverge

A picture might help...

Here is the current pointer list before the snapshot...

XXXXXXXXXXXXXXXXXXXXXXXXXXX

Then you take a snapshot...

Current pointer list

XXXXXXXXXXXXXXXXXXXXXXXXXXX

Snapshot pointer list

XXXXXXXXXXXXXXXXXXXXXXXXXXX

Next several changes come into the database which cause a physical block
changes in the filesystem...

Current pointer list

XXAAAAAAXXXXXXXXXXXZZZZXXXX

Snapshot pointer list

XXXXXXXXXXXXXXXXXXXXXXXXXXX


At this point, the majority of the pointer lists still point to the same
physical blocks in both lists... However, the current pointer list is
the only one that changes... This means that only one write was
performed (and the pointer list was updated)...

At least that's the way it was explained to me... I could have been lied
to...

:-)

Tim
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Re: Falconstor/IPStor with Clariion, EMC Timefinder with Symmetrix

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