Re: does postgresql backup require additional space on disk - Mailing list pgsql-general

On 5/13/19 12:33 PM, Julie Nishimura wrote:
> We have added new disk volume and about to introduce new tablespace 
> using this additional disk space. After that, I am going to alter all 
> user dbs (and template db as well) by runnig the following command:

Not sure why you need to move the template db's they sure not take much 
space?

More comments below.

> 
> CREATE TABLESPACE vol4
> OWNER postgres
> LOCATION '/data/vol4';
> 
> ALTER DATABASE user_db_1
> SET default_tablespace = 'vol4';
> 
> ...for all dbs..
> 
> We have more than 70 different databases (the entire server is about 20 
> tb). However, for the largest dbs (16tb, 4 tb, and 3 tb), we do not have 
> valid backups, unfortunately. So, we were debating if we need to run 
> backups first before introducing all these changes. But we have only 1% 
> left (about 200 gb).

I have no experience with a setup of this size, someone else will have 
to provide real world advice. I would say taking a backup before 
preceding is a good idea. Pretty sure time is going to be as much an 
issue as space. This would apply to below also. The question is what 
options you have to bring cluster or databases down? In particular for 
below as:

https://www.postgresql.org/docs/11/sql-alterdatabase.html
"The fourth form changes the default tablespace of the database. Only 
the database owner or a superuser can do this; you must also have create 
privilege for the new tablespace. This command physically moves any 
tables or indexes in the database's old default tablespace to the new 
tablespace. The new default tablespace must be empty for this database, 
and no one can be connected to the database. Tables and indexes in 
non-default tablespaces are unaffected."

> 
> What would you suggest? How would you classify the risk of this 
> operation (creating new tablespace and altering dbs to use it)?
> 
> Thanks,
> Julie
> 
> ------------------------------------------------------------------------
> *From:* Adrian Klaver <adrian.klaver@aklaver.com>
> *Sent:* Monday, May 13, 2019 11:19 AM
> *To:* Julie Nishimura; pgsql-general@lists.postgresql.org; pgsql-general
> *Subject:* Re: does postgresql backup require additional space on disk
> On 5/13/19 10:59 AM, Julie Nishimura wrote:
>> Hello,
>> we are almost out of disk space on one of our servers (99% full). If we 
>> run pg_dump to a diff location, does it require any additional disk 
>> space on our current server? I am asking, because on some other 
>> software, a backup might open transaction which keeps growing and 
>> eventually consume all space, keeping tran open for the backup duration. 
>> Please clarify? Thanks
> 
> A pg_dump is a point in time snapshot of the database, so if the cluster
> is running then it will advance past the dump snapshot. If the cluster
> is not active(close off connections to all but pg_dump) then pg_dump
> will be the only transaction.
> 
> I think the first thing to ask is what you are trying to achieve?
> 
> -- 
> Adrian Klaver
> adrian.klaver@aklaver.com


-- 
Adrian Klaver
adrian.klaver@aklaver.com