Re: open database on read only file system

Well, I mean, yeah. You're (presumably) trying to test changes before going live? But you're changing the code to run on an RO system? That's not an apples-to-apples test, and (while unlikely) your results could be different in production.

If **I** were doing this? I would set up replication, get it in sync to my internal test server, break the replication, promote the slave and run my tests.

That's a 1:1.

That would make me the most confident.

-------- Original message --------
From: xujian <jamesxu(at)outlook(dot)com>
Date: 10/07/2016 4:46 PM (GMT-06:00)
To: Scott Whitney <scott(at)journyx(dot)com>, pgsql-admin(at)postgresql(dot)org
Subject: RE: [ADMIN] open database on read only file system

Thanks Scott. Do you know if there is any way to support read-only FS? for instance patch or extension.

I noticed the error is from file md.c
========
/*
* mdopen() -- Open the specified relation.
*
* Note we only open the first segment, when there are multiple segments.
*
* If first segment is not present, either ereport or return NULL according
* to "behavior". We treat EXTENSION_CREATE the same as EXTENSION_FAIL;
* EXTENSION_CREATE means it's OK to extend an existing relation, not to
* invent one out of whole cloth.
*/
static MdfdVec *
mdopen(SMgrRelation reln, ForkNumber forknum, int behavior)
========

so I changed the file open mode from O_RDWR to O_RDONLY
fd = PathNameOpenFile(path, O_RDONLY | PG_BINARY, 0600);

and it works. I can open the user database, and query table now. Do you think if there is any potential problem of this change?
I just need to open database on read-only file system, and no need any changes on the cluster. thanks

James

________________________________
From: scott(at)journyx(dot)com
To: jamesxu(at)outlook(dot)com; pgsql-admin(at)postgresql(dot)org
Subject: Re: [ADMIN] open database on read only file system
Date: Fri, 7 Oct 2016 20:06:09 +0000

To my knowledge, it is not, no.

The FS must be read/write.

What I would do in this situation is to migrate it to a test/new system where no one can access it and bring it up r/w.

Or set up streaming replication. That gives you select-only.

________________________________
From: pgsql-admin-owner(at)postgresql(dot)org <pgsql-admin-owner(at)postgresql(dot)org> on behalf of xujian <jamesxu(at)outlook(dot)com>
Sent: Friday, October 7, 2016 2:43 PM
To: pgsql-admin(at)postgresql(dot)org
Subject: [ADMIN] open database on read only file system

Hi,
Could you please let me know if it is possible to open database on read-only file system?

I have a psotgresql 9.6 cluster, PGDATA folder is on READ-WRITE volume(system volume), user database is created on a table space which is located on another volume(data volume). Now we set the data volume to read only mode, before we changed the mode, I also disabled the auto vacuum, run checkpoint,
and vacuum free. We reboot the service, everything was fine until we tried to access the database.
===============
postgres=# \c userdb;
FATAL: could not open file "pg_tblspc/16384/PG_9.6_201608131/16468/2662": Read-only file system
===============

we enabled the debug mode, in log file, we saw
===============
FATAL: 42501: could not open file "pg_tblspc/16384/PG_9.6_201608131/16468/2662": Read-only file system
LOCATION: mdopen, md.c:609
===============

Does anyone know if there is anyway to run database on read-only file system? thanks

James

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