Re: Enabling Checksums

On 3/20/13 8:41 AM, Bruce Momjian wrote:
> On Mon, Mar 18, 2013 at 01:52:58PM -0400, Bruce Momjian wrote:
>> I assume a user would wait until they suspected corruption to turn it
>> on, and because it is only initdb-enabled, they would have to
>> dump/reload their cluster.  The open question is whether this is a
>> usable feature as written, or whether we should wait until 9.4.
>>
>> pg_upgrade can't handle this because the old/new clusters would have the
>> same catalog version number and the tablespace directory names would
>> conflict.  Even if they are not using tablespaces, the old heap/index
>> files would not have checksums and therefore would throw an error as
>> soon as you accessed them.  In fact, this feature is going to need
>> pg_upgrade changes to detect from pg_controldata that the old/new
>> clusters have the same checksum setting.
>
> A few more issues with pg_upgrade: if we ever decide to change the
> checksum calculation in a later major release, pg_upgrade might not work
> because of the checksum change but could still work for users who don't
> use checksums.
>
> Also, while I understand why we have to set the checksum option at
> initdb time, it seems we could enable users to turn it off after initdb
> --- is there any mechanism for this?
>
> Also, if a users uses checksums in 9.3, could they initdb without
> checksums in 9.4 and use pg_upgrade?  As coded, the pg_controldata
> checksum settings would not match and pg_upgrade would throw an error,
> but it might be possible to allow this, i.e. you could go from checksum
> to no checksum initdb clusters, but not from no checksum to checksum.  I
> am wondering if the patch should reflect this.

If the docs don't warn about this, they should, but I don't think it's the responsibility of this patch to deal with that problem. The reason I don't believe this patch should deal with it is because that is a known, rather serious, limitation of pg_upgrade. It's something about pg_upgrade that just needs to be fixed, regardless of what patches might make the situation worse.