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* Which postgres process cleans up deprecated datafiles
@ 2022-03-17 17:43 Loles <lolesft@gmail.com>
0 siblings, 1 reply; 2+ messages in thread
From: Loles @ 2022-03-17 17:43 UTC (permalink / raw)
To: pgsql-admin
Hi Team!
Teaching class a question has arisen.. about datafiles that remain after a
truncate or vacuum full operation.
See a simple example before asking my question.
16470 is the filenode corresponding to the test table:
# select relname, relfilenode from pg_class where relname = 'test';
relname | relfilenode
---------+-------------
test | 16470
(1 row)
Exists in the filesystem:
$ ls -lh /postgresql/datos/base/12738/ | grep 16470
-rw------- 1 xxxxxxxxxxxxxx xxxxxxxxxxxxxx 50M mar 17 18:08 16470
I perform a truncate command on the table:
# truncate table test;
TRUNCATE TABLE
And the file still exists with size 0 bytes.
$ ls -lh /postgresql/datos/base/12738/ | grep 16470
-rw------- 1 xxxxxxxxxxxxxx xxxxxxxxxxxxxx 0 mar 17 18:20 16470
But a truncate command, PostgreSQL drops table and creates a new one. Now,
filenode is 16476 and it also has 0 bytes.
# select relname, relfilenode from pg_class where relname = 'test';
relname | relfilenode
---------+-------------
test | 16476
(1 row)
$ ls -lh /postgresql/datos/base/12738/ | grep 16476
-rw------- 1 xxxxxxxxxxxxxx xxxxxxxxxxxxxx 0 mar 17 18:20 16476
Ok, but... after an indeterminate amount of time, the datafile 16470
disappeared.
Now the question.
What postgres process, or system, identify and delete datafiles that have
become unassociated? The truncate operation doesn't appear to be.. the
datafile has existed for some time after the truncate command execution
finished.
Thanks and Greetings!
^ permalink raw reply [nested|flat] 2+ messages in thread
* Re: Which postgres process cleans up deprecated datafiles
@ 2022-03-17 18:40 Tom Lane <tgl@sss.pgh.pa.us>
parent: Loles <lolesft@gmail.com>
0 siblings, 0 replies; 2+ messages in thread
From: Tom Lane @ 2022-03-17 18:40 UTC (permalink / raw)
To: Loles <lolesft@gmail.com>; +Cc: pgsql-admin
Loles <lolesft@gmail.com> writes:
> What postgres process, or system, identify and delete datafiles that have
> become unassociated? The truncate operation doesn't appear to be.. the
> datafile has existed for some time after the truncate command execution
> finished.
The zero-length file is a placeholder to ensure that that relfilenode
number doesn't get re-used right away. It'll get cleaned up during
the next checkpoint.
regards, tom lane
^ permalink raw reply [nested|flat] 2+ messages in thread
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