Re: [GENERAL] Unlogged Crash Detection

2017-08-29 Thread Michael Paquier 
On Tue, Aug 29, 2017 at 11:09 PM, Andres Freund  wrote:
> Huh, but that's not particularly meaningful, is it? That'll just as well
> be the case for a freshly created relation, no?

I have assumed that the OP has some control on the timing of the
relations, using an event trigger for example. There is no perfect
method I am afraid.
-- 
Michael


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

Re: [GENERAL] Unlogged Crash Detection

2017-08-29 Thread Andres Freund 
On 2017-08-29 20:19:52 +0900, Michael Paquier wrote:
> On Tue, Aug 29, 2017 at 6:06 PM, Gersner  wrote:
> > I see, interesting.
> 
> Please do not top-post. This is not the recommended way of dealing
> with threads on this mailing list.
> 
> > We have lots of unlogged tables, upon a crash we want to create a
> > feedback/alert that data disappeared.
> >
> > Not very familiar with the internal structure, but is it possible to
> > identify if the current table is the INIT_FORKNUM?
> 
> Using pg_relation_filepath, you can know the path to a relation file
> on disk. So a simple idea would be to use pg_read_binary_file with the
> path of the file and the path of the init fork, which is suffixed with
> "_init", and then a comparison between both. If the data read is the
> same, the relation has been untouched.

Huh, but that's not particularly meaningful, is it? That'll just as well
be the case for a freshly created relation, no?

- Andres


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

Re: [GENERAL] Unlogged Crash Detection

2017-08-29 Thread Michael Paquier 
On Tue, Aug 29, 2017 at 6:06 PM, Gersner  wrote:
> I see, interesting.

Please do not top-post. This is not the recommended way of dealing
with threads on this mailing list.

> We have lots of unlogged tables, upon a crash we want to create a
> feedback/alert that data disappeared.
>
> Not very familiar with the internal structure, but is it possible to
> identify if the current table is the INIT_FORKNUM?

Using pg_relation_filepath, you can know the path to a relation file
on disk. So a simple idea would be to use pg_read_binary_file with the
path of the file and the path of the init fork, which is suffixed with
"_init", and then a comparison between both. If the data read is the
same, the relation has been untouched. Note that you can only do such
a thing as a superuser, and that the data is read from disk, not from
shared buffers. So that's not perfect, but it give an indication.
-- 
Michael


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

Re: [GENERAL] Unlogged Crash Detection

2017-08-29 Thread Gersner 
I see, interesting.

We have lots of unlogged tables, upon a crash we want to create a
feedback/alert that data disappeared.

Not very familiar with the internal structure, but is it possible to
identify if the current table is the INIT_FORKNUM?

Gersner

On Tue, Aug 29, 2017 at 11:27 AM, Michael Paquier  wrote:

> On Tue, Aug 29, 2017 at 5:17 PM, Gersner  wrote:
> > Is there a reliable way to distinguish between an empty unlogged table
> to an
> > unlogged table which has been truncated due to a crash?
>
> Why do you want to make such a difference? At the beginning of a crash
> recovery all the, the existing relation files of unlogged tables are
> all removed, and replaced by the init forknum which represents their
> initial state. You can see by yourself ResetUnloggedRelations &
> friends in reinit.c.
> --
> Michael
>

Re: [GENERAL] Unlogged Crash Detection

2017-08-29 Thread Michael Paquier 
On Tue, Aug 29, 2017 at 5:17 PM, Gersner  wrote:
> Is there a reliable way to distinguish between an empty unlogged table to an
> unlogged table which has been truncated due to a crash?

Why do you want to make such a difference? At the beginning of a crash
recovery all the, the existing relation files of unlogged tables are
all removed, and replaced by the init forknum which represents their
initial state. You can see by yourself ResetUnloggedRelations &
friends in reinit.c.
-- 
Michael


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