Search Postgresql Archives

Re: checkpoint clarifications needed

[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

 



Tom DalPozzo wrote:

> Hi,
> so let's suppose that the WAL is:
> LSN 10: start transaction 123
> LSN 11: update tuple 100
>    checkpoint position here (not a record but just for understanding)
> LSN 12: update tuple 100
> LSN 13: update tuple 100
> LSN 14: checkpoint record ( postion=11)
> LSN 15: update tuple 100
> and that the system crashes now, before ending to write all the
> transaction's recs to the WAL  (other updates and commit record missing).
> 
> At the replay, starting from LSN 12, the entire page we had at LSN 11 is
> written to the disk, though carrying inconsistent data.
> Then we can even replay up to the end of WAL but always getting
> inconsistent data.
> BUT, you say, as the tuple is not commited in the WAL, only the old version
> of the tuple will be visible? Right?

Yes -- all the updated tuples are invisible because the commit record
for transaction 123 does not appear in wal.  A future VACUUM will remove
all those tuples.  Note that precisely for this reason, the original
version of the tuple had not been removed yet.

-- 
Álvaro Herrera                https://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services


-- 
Sent via pgsql-general mailing list (pgsql-general@xxxxxxxxxxxxxx)
To make changes to your subscription:
http://www.postgresql.org/mailpref/pgsql-general



[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]
[Index of Archives]     [Postgresql Jobs]     [Postgresql Admin]     [Postgresql Performance]     [Linux Clusters]     [PHP Home]     [PHP on Windows]     [Kernel Newbies]     [PHP Classes]     [PHP Books]     [PHP Databases]     [Postgresql & PHP]     [Yosemite]
  Powered by Linux