Re: Potential data loss of 2PC files

From: Heikki Linnakangas <hlinnaka(at)iki(dot)fi>
To: Andres Freund <andres(at)anarazel(dot)de>, Michael Paquier <michael(dot)paquier(at)gmail(dot)com>
Cc: Robert Haas <robertmhaas(at)gmail(dot)com>, PostgreSQL mailing lists <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: Potential data loss of 2PC files
Date: 2017-01-30 13:52:59
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On 12/27/2016 01:31 PM, Andres Freund wrote:
> On 2016-12-27 14:09:05 +0900, Michael Paquier wrote:
>> On Fri, Dec 23, 2016 at 3:02 AM, Andres Freund <andres(at)anarazel(dot)de> wrote:
>>> Not quite IIRC: that doesn't deal with file size increase. All this would be easier if hardlinks wouldn't exist IIUC. It's basically a question whether dentry, inode or contents need to be synced. Yes, it sucks.
>> I did more monitoring of the code... Looking at unlogged tables and
>> empty() routines of access methods, isn't there a risk as well for
>> unlogged tables? mdimmedsync() does not fsync() the parent directory
>> either!
> But the files aren't created there, so I don't generally see the
> problem. And the creation of the metapages etc. should be WAL logged
> anyway. So I don't think we should / need to do anything special
> there. You can argue however that we wouldn't necessarily fsync the
> parent directory for the file creation, ever. But that'd be more
> smgrcreate's responsibility than anything.

So, if I understood correctly, the problem scenario is:

1. Create and write to a file.
2. fsync() the file.
3. Crash.
4. After restart, the file is gone.

If that can happen, don't we have the same problem in many other places?
Like, all the SLRUs? They don't fsync the directory either.

Is unlink() guaranteed to be durable, without fsyncing the directory? If
not, then we need to fsync() the directory even if there are no files in
it at the moment, because some might've been removed earlier in the
checkpoint cycle.

- Heikki

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