RE: split block (torn page) problem
- From: Laimutis.Nedzinskas@xxxxxx
- To: "'ORACLE-L'" <oracle-l@xxxxxxxxxxxxx>
- Date: Thu, 8 Dec 2011 15:36:56 +0200
>you need backups
Absolutely. I can only stress once and again: strictly speaking you need
backups to recover from crashes (redo corruption is not considered here.)
What oracle calls crash-recovery recovers most of a time but I personally
had one case of development server crashing(no UPS) and database not
starting due to failed crash recovery.
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From: "Mark W. Farnham" <mwf@xxxxxxxx>
To: <Laimutis.Nedzinskas@xxxxxx>, "'ORACLE-L'"
<oracle-l@xxxxxxxxxxxxx>
Date: 2011.12.08 15:19
Subject: RE: split block (torn page) problem
The problem is at a more fundamental level. Disk sector sizes are mostly
512
and some 4096 bytes usable.
Whenever the logical block size in Oracle is bigger (almost always now),
there is a possibility of the implementation of Oracle's write being
promised by the OS, but not in fact truly being complete yet.
Whether you call that an OS or hardware failure, it is indeed a failure
that
can happen, ergo the need for backups combined with a valid redo stream to
ensure recoverability.
This is qualitatively different from "physical hot backups", where Oracle's
dbwr process might issue a 8KB block write, be waiting for confirmation,
and
at the same time an OS utility operating at the native disk sizes and
boundaries sweeps up physical sectors containing part of the logical Oracle
block already written and other physical sectors still containing an older
vintage. No hardware error has happened, but unless you properly had the
tablespace of which the file or raw component is a part in "backup mode"
the
redo stream might not have the information to reconstruct a valid version
of
the block. No media error or OS error is required in this case, but it is
an
opportunity for pilot error.
RMAN avoids this particular problem by using Oracle logical writes instead
of an OS utility.
But in the case of an OS ordered physical write being partially complete,
that is an OS or hardware failure, whether it originates from a race
condition with power loss or failure of the storage substrate. So even if
the pilot has done everything correctly and there is no bug in Oracle, you
may need to do a media recovery.
The reason I'm beating on this is that the commentary can be read to
suggest
that battery backed storage or ASM might remove the need for backups. That
is most certainly NOT what Riyaj was suggesting by his remark on
probabilities. It is probably not what you mean either, so please treat
this
as clarification for newbies rather than some sort of flame.
You need backups to recovery from media errors. Partial writes from the OS
layer are rightly considered media errors and can sometimes result from an
abrupt power outage or other crash.
Normally an Oracle warm restart applies the proper deltas from the redo
stream's online redo logs to bring current the blocks that were lost dirty
in the buffer cache at the time of the crash. Unless there is a corrupt
block already in the file instance recovery is operating on, that should
just work (or else you've found a bug.)
-----Original Message-----
From: oracle-l-bounce@xxxxxxxxxxxxx [mailto:oracle-l-bounce@xxxxxxxxxxxxx]
On Behalf Of Laimutis.Nedzinskas@xxxxxx
Sent: Thursday, December 08, 2011 6:52 AM
To: ORACLE-L
Subject: Re: split block (torn page) problem
> With ASM, file system caching is not involved and write calls operate
> on
devices. So, probability is much less with ASM.
It can be asked the other way arround. It's more than common that personal
computers are crashed once and again. However disk corruptions resulting
from crashes either go undetected or ... they happen very rarely. How big
is
the probability of disk devices corrupting blocks on writes?
My understanding would be that:
if we take battery backed disk devices then disk can pretty much guarantee
that block write is atomic (fails or succeeds.) Messaging from server to
disk device is no problem provided some minimal care is taken to check (or
even cheksum) that disk receives a whole block.
It's interesting how big is the probability of disk block corruption for
non-battery backed disks?
All that does not prevent torn(fractured) database page consisting of many
blocks of course. Unless the path from server process to disk device is
protected to deliver the whole db page or nothing.
brgds, Laimis N
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From: Riyaj Shamsudeen <riyaj.shamsudeen@xxxxxxxxx>
To: Laimutis.Nedzinskas@xxxxxx
Cc: ORACLE-L <oracle-l@xxxxxxxxxxxxx>
Date: 2011.12.08 07:24
Subject: Re: split block (torn page) problem
Hi
crash recovery applies redo records to roll forward the changes. If a
modified block is not written yet to the disk, that is okay, as redo
records
from the log files can be used to replay the changes.
But, if the block is not written properly or fractured, then the crash
recovery will raise corruption errors and can't correct the corruption.
In enterprise servers, server reboots does not cause any corruption
(usually). From oracle point of view, a buffer is filled with block image
and I/O submitted to the OS. If the OS splits the call in to smaller chunks
(say 4K) and writes with two atomic calls underneath the write system call
(and that first 4K chunk succeeded, second 4K chunk write did not succeed),
it is possible for the corruption to occur, but it is a corner case and you
must be very unfortunate :-)
With ASM, file system caching is not involved and write calls operate on
devices. So, probability is much less with ASM.
HTH
Cheers
Riyaj Shamsudeen
Principal DBA,
Ora!nternals - http://www.orainternals.com - Specialists in Performance,
RAC and EBS
Blog: http://orainternals.wordpress.com
OakTable member http://www.oaktable.com and Oracle ACE Director
Co-author of the books: Expert Oracle
Practices<http://tinyurl.com/book-expert-oracle-practices/:>,
Pro Oracle SQL, Expert PL/SQL
Practices<http://tinyurl.com/book-expert-plsql-practices>
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