On 04/01/16 18:12, Andres Freund wrote:
Pages containing data of unlogged tables aren't ever flushed to disk
unless
a) a shutdown checkpoint is performed
b) a buffer containing data from an unlogged table is used for something
else
c) the database being copied is the the source of a CREATE DATABASE .. TEMPLATE
Hence, if there's an unclean shutdown, there's absolutely no guarantee
about the on-disk state of unlogged tables. Even if they haven't been
modified in ages - there could have been many many dirty pages in shared
buffers when crashing.
Always flushing dirty pages of unlogged tables at checkpoint would
greatly increase the overhead for memory resident, write heavy workloads
that use unlogged tables.
If there was a command to flush a specific unlogged table to disk it
would work around all these issues no? Perhaps if you marked the table
as read only at the same time it would flush it to disk and ensure no
more data could be written to it eg (ALTER TABLE ... SET READ ONLY on an
unlogged table would flush + not truncate after crash). In our case this
would be great as we want to use these as permanent tables for speed;
but after an initial data dump we don't change the data again so we
could just do this at the end of the import process.
Mark
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