Re: [patch] Fix checksum verification in base backups for zero page headers

On 22.10.2020 04:25, Michael Paquier wrote:
> On Thu, Oct 22, 2020 at 12:47:03AM +0300, Anastasia Lubennikova wrote:
>> We can also read such pages via shared buffers to be 100% sure.
> Yeah, but this has its limits as well.  One can use
> ignore_checksum_failure, but this can actually be very dangerous as
> you can finish by loading into shared buffers a page that has a header
> thought as sane but with a large portion of its page randomly
> corrupted, spreading corruption around and leading to more fancy
> logic failures in Postgres, with more panic from customers.  Not using
> ignore_checksum_failure is safer, but it makes an analyze of the
> damages for a given file harder as things would stop at the first
> failure of a file with a seqscan.  pg_prewarm can help here, but
> that's not the goal of the tool to do that either.
I was thinking about applying this only to pages with LSN > startLSN.

Most of such pages are valid and already in memory, because they were 
changed just recently, so no need for pg_prewarm here. If such LSN 
appeared because of a data corruption, page verification from inside 
ReadBuffer() will report an error first. In proposed function, we can 
handle this error in any fashion we want. Something like:

if (PageGetLSN(page) > startptr)
{
     if (!read_page_via_buffercache())

         //throw a warning about corrupted page
         //handle checksum error as needed
     else
         //page is valid. No worries
}

-- 
Anastasia Lubennikova
Postgres Professional: http://www.postgrespro.com
The Russian Postgres Company