Alvaro Herrera wrote on 8/3/20 2:34 PM:On 2020-Aug-03, Ben Chobot wrote:
Alvaro Herrera wrote on 8/3/20 12:34 PM:
On 2020-Aug-03, Ben Chobot wrote:
Yep. Looking at the ones in block 6501,
rmgr: Btree len (rec/tot): 72/ 72, tx: 76393394, lsn:
A0A/AB2C43D0, prev A0A/AB2C4378, desc: INSERT_LEAF off 41, blkref #0: rel
16605/16613/60529051 blk 6501
rmgr: Btree len (rec/tot): 72/ 72, tx: 76396065, lsn:
A0A/AC4204A0, prev A0A/AC420450, desc: INSERT_LEAF off 48, blkref #0: rel
16605/16613/60529051 blk 6501
My question was whether the block has received the update that added the
item in offset 41; that is, is the LSN in the crashed copy of the page
equal to A0A/AB2C43D0? If it's an older value, then the write above was
lost for some reason.
How do I tell?
You can use pageinspect's page_header() function to obtain the page's
LSN. You can use dd to obtain the page from the file,
dd if=16605/16613/60529051 bs=8192 count=1 seek=6501 of=/tmp/page.6501
If I use skip instead of seek....
then put that binary file in a bytea column, perhaps like
create table page (raw bytea);
insert into page select pg_read_binary_file('/tmp/page');
and with that you can run page_header:
create extension pageinspect;
select h.* from page, page_header(raw) h;
lsn | checksum | flags | lower | upper | special | pagesize | version | prune_xid
--------------+----------+-------+-------+-------+---------+----------+---------+-----------
A0A/99BA11F8 | -215 | 0 | 180 | 7240 | 8176 | 8192 | 4 | 0
As I understand what we're looking at, this means the WAL stream was assuming this page was last touched by A0A/AB2C43D0, but the page itself thinks it was last touched by A0A/99BA11F8, which means at least one write to the page is missing?
