On 30 July 2015 at 15:24, Heikki Linnakangas <hlinnaka@iki.fi> wrote:
On 07/30/2015 04:26 PM, Alexander Korotkov wrote:
Also, I think it's possible to migrate to 64-bit XIDs without breaking pg_upgrade. Old tuples can be leaved with 32-bit XIDs while new tuples would be created with 64-bit XIDs. We can use free bits in t_infomask2 to distinguish old and new formats.
I think we should move to 64-bit XIDs in in-memory structs snapshots, proc array etc. And expand clog to handle 64-bit XIDs. But keep the xmin/xmax fields on heap pages at 32-bits, and add an epoch-like field to the page header so that logically the xmin/xmax fields on the page are 64 bits wide, but physically stored in 32 bits. That's possible as long as no two XIDs on the same page are more than 2^31 XIDs apart. So you still need to freeze old tuples on the page when that's about to happen, but it would make it possible to have more than 2^32 XID transactions in the clog. You'd never be forced to do anti-wraparound vacuums, you could just let the clog grow arbitrarily large.
This is a good scheme, but it assumes, as you say, that you can freeze tuples that are more than 2^31 xids apart. That is no longer a safe assumption on high transaction rate systems with longer lived snapshots.
There is a big downside to expanding xmin/xmax to 64 bits: it takes space. More space means more memory needed for caching, more memory bandwidth, more I/O, etc.
My feeling is that the overhead will recede in time. Having a nice, simple change to remove old bugs and new would help us be more robust.
But let's measure the overhead before we try to optimize it away.
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