question about checksum in 9.3
Hello, I am in the process of planning a 9.3 migration of postgres and I am
curious about the checksum features available. In my test 9.3 instance it
seemed like this feature provides a log entry of the exact database/oid of
the corrupt object when it is accessed, but not much else. I can't find
much documentation on anything else this feature provides.
Is there a built-in method of scanning the server to check for corruption
or will I have to wait for a corrupt object to be accessed to see the log
entry? Is there a relation that stores last checksum status or anyway of
reporting on what objects are identified by postgres as corrupt or not
corrupt?
Are there any other features of the checksum I am missing besides the log
entry?
Thanks
Mike
On 01/13/2014 08:50 AM, Mike Broers wrote:
Hello, I am in the process of planning a 9.3 migration of postgres and I
am curious about the checksum features available. In my test 9.3
instance it seemed like this feature provides a log entry of the exact
database/oid of the corrupt object when it is accessed, but not much
else. I can't find much documentation on anything else this feature
provides.Is there a built-in method of scanning the server to check for
corruption or will I have to wait for a corrupt object to be accessed to
see the log entry? Is there a relation that stores last checksum status
or anyway of reporting on what objects are identified by postgres as
corrupt or not corrupt?Are there any other features of the checksum I am missing besides the
log entry?
I could not find anything and the reasons for that would seem to a
result of this sentence:
"The checksum is not valid at all times on a data page!!"
from the below where the gory details are laid out:
src/backend/storage/page/README
Thanks
Mike
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Adrian Klaver
adrian.klaver@gmail.com
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On Tue, Jan 14, 2014 at 1:50 AM, Mike Broers <mbroers@gmail.com> wrote:
Hello, I am in the process of planning a 9.3 migration of postgres and I am
curious about the checksum features available. In my test 9.3 instance it
seemed like this feature provides a log entry of the exact database/oid of
the corrupt object when it is accessed, but not much else. I can't find
much documentation on anything else this feature provides.
Few things:
- The only way to know if a server is using data checksums is to use
pg_controldata.
- Be aware as well of the potential performance impact on your CPU,
checksums are checked each time a page is read, and recalculated each
time a page is updated.
- ignore_checksum_failure can be used to ignore failures. Don't use
that on a production system.
Is there a built-in method of scanning the server to check for corruption or
will I have to wait for a corrupt object to be accessed to see the log
entry?
You can as well access manually tables with some for example
sequential scan to check if blocks are broken or not.
Is there a relation that stores last checksum status or anyway of
reporting on what objects are identified by postgres as corrupt or not
corrupt?
No, you could build one though with a background worker that scans
relation pages and registers that failing blocks.
Are there any other features of the checksum I am missing besides the log
entry?
9.4 has a new GUC parameter called data_checksums that allow you to
check with a psql client if checksums are used on a server.
Regards,
--
Michael
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Thanks for the responses, that clarifies the checksum feature for me.
FWIW, my pgbench tests between a 9.2 server and a 9.3 server with checksum
showed very similar performance characteristics and system resource
utilization. Im going to set up another load test with our actual
application to see if that reveals any noticeable performance difference.
Thanks again
Mike
On Mon, Jan 13, 2014 at 7:11 PM, Michael Paquier
<michael.paquier@gmail.com>wrote:
Show quoted text
On Tue, Jan 14, 2014 at 1:50 AM, Mike Broers <mbroers@gmail.com> wrote:
Hello, I am in the process of planning a 9.3 migration of postgres and I
am
curious about the checksum features available. In my test 9.3 instance
it
seemed like this feature provides a log entry of the exact database/oid
of
the corrupt object when it is accessed, but not much else. I can't find
much documentation on anything else this feature provides.Few things:
- The only way to know if a server is using data checksums is to use
pg_controldata.
- Be aware as well of the potential performance impact on your CPU,
checksums are checked each time a page is read, and recalculated each
time a page is updated.
- ignore_checksum_failure can be used to ignore failures. Don't use
that on a production system.Is there a built-in method of scanning the server to check for
corruption or
will I have to wait for a corrupt object to be accessed to see the log
entry?You can as well access manually tables with some for example
sequential scan to check if blocks are broken or not.Is there a relation that stores last checksum status or anyway of
reporting on what objects are identified by postgres as corrupt or not
corrupt?No, you could build one though with a background worker that scans
relation pages and registers that failing blocks.Are there any other features of the checksum I am missing besides the log
entry?9.4 has a new GUC parameter called data_checksums that allow you to
check with a psql client if checksums are used on a server.Regards,
--
Michael
On 13/01/14 17:50, Mike Broers wrote:
Is there a built-in method of scanning the server to check for
corruption or will I have to wait for a corrupt object to be accessed to
see the log entry?
This presentation:
http://www.youtube.com/watch?v=TovZ0lb16-Q
suggests pg_filedump.
Torsten
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