Re: Data Mirroring on two data centers -- How to use ASM ?
- From: Tanel Põder <tanel.poder.003@xxxxxxx>
- To: "Edgar Chupit" <chupit@xxxxxxxxx>, <oracle-l@xxxxxxxxxxxxx>
- Date: Sun, 21 May 2006 20:39:45 +0800
I wondered if anyone brings the HARD up ;)
I haven't really seen nor heard anybody using it. HARD basically means that
storage subsystems know where the checksum bytes and relative data block
address bytes reside within a block plus few extra trivial checks. However
the situation isn't as bright as Oracle's doc states.
There are several limitations and fundamental problems, like what to do with
detected blocks which checksum bytes don't match the content of the block.
One option is to write the block to disk and alert storage administrator/DBA
for post-diagnostics. This means that the corruption will get replicated to
DR site - and that's a problem.
Other option would be to reject the corrupt write, causing Oracle to retry
it. If we are lucky and the error is transient, the processing continues,
but if not, then Oracle has to take the datafile offline. And if the
datafile happens to be in system or undo tablespace, your services (and
instances) are pretty much dead.
EMC offers a discard option as well, that the corrupt block write are just
silently thrown away, but that's the last thing we want to happen..
Another issue is that there must be no abstraction layers such filesystems
between storage and Oracle. You have to use RAW devices and have to make
sure that the blocks written to storage aren't moved around in higher layers
as the correct block address checking relies on comparing the physical
address on raw volume to the RDBA stored within block header.
Read more: http://www.emc.com/pdf/techlib/EMCDoubleChecksum.pdf
Also, it seems that the interest in HARD initiative has slowly died, the
last certified DB version is 9.2:
http://www.oracle.com/technology/deploy/availability/htdocs/vendors_hard.html
Kevin probably knows stories from the field, if anyone actually has used
successfully... Kevin, wake up ;)
Tanel.
----- Original Message -----
From: "Edgar Chupit" <chupit@xxxxxxxxx>
To: <oracle-l@xxxxxxxxxxxxx>
Cc: <tanel.poder.003@xxxxxxx>
Sent: Sunday, May 21, 2006 7:59 PM
Subject: Re: Data Mirroring on two data centers -- How to use ASM ?
To solve this kind of problems, oracle has created Oracle Hardware
Assisted Resilient Data (HARD) Initiative, and according to the
http://www.oracle.com/technology/deploy/availability/htdocs/HARD.html
all large storage providers are members of this initiative, so „smart"
storage does know about Oracle block format and can catch block
corruptions, before they are written to disk.
On 5/21/06, Tanel Põder <tanel.poder.003@xxxxxxx> wrote:
Btw, one more thing if talking about redolog level replication versus
storage replication is that as storage doesn't normally know anything
about Oracle's block formats then it doesn't have any choice but replicate
all corruptions to remote site as well. Oracle's redo apply mechanism
would detect most of such issues...
Tanel.
--
Best regards,
Edgar Chupit
callto://edgar.chupit
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