Alvaro Herrera wrote:
A customer of ours has been having trouble with corrupted data for some
time. Of course, we've almost always blamed hardware (and we've seen
RAID controllers have their firmware upgraded, among other actions), but
the useful thing to know is when corruption has happened, and where.
So we've been tasked with adding CRCs to data files.
The idea is that these CRCs are going to be checked just after reading
files from disk, and calculated just before writing it. They are
just a protection against the storage layer going mad; they are not
intended to protect against faulty RAM, CPU or kernel.
This has been suggested before, and the usual objection is precisely
that it only protects from errors in the storage layer, giving a false
sense of security.
Doesn't some filesystems include a per-block CRC, which would achieve
the same thing? ZFS?
This code would be run-time or compile-time configurable. I'm not
absolutely sure which yet; the problem with run-time is what to do if
the user restarts the server with the setting flipped. It would have
almost no impact on users who don't enable it.
Yeah, seems like it would need to be compile-time or initdb-time
configurable.
The implementation I'm envisioning requires the use of a new relation
fork to store the per-block CRCs. Initially I'm aiming at a CRC32 sum
for each block. FlushBuffer would calculate the checksum and store it
in the CRC fork; ReadBuffer_common would read the page, calculate the
checksum, and compare it to the one stored in the CRC fork.
Surely it would be much simpler to just add a field to the page header.
--
Heikki Linnakangas
EnterpriseDB http://www.enterprisedb.com
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