Re: journaled FS and and WAL

So, as for the data content of the WAL file, I see that no more page
will be allocated. I wonder if during a crash, strange things can still
happen at disk level however, in particular in SSD devices; on these
things we have no control, and perhaps journaling helps?
As for the metadata, if during a crash it's flushed (with fdatasync,
only when the FS decides to do that), can anything bad happen without
journaling?

Third, let's suppose that the WAL can't get corrupted. When the system
flushes data pages to the disk according to the WAL content, if there is
a crash, am I sure that tables files old pages and /or their metadata,
inode.... can't get corrupted?
If that, there is no possibility to reconstruct the things, even through
the WAL. Even in this case, perhaps journaling helps.

I don't mind about performance but I absolutely mind about reliability,
so I was thinking about the safest setting of linux FS and postgresql I
can use.
Thanks!
Pupillo






Il 15/10/2016 07:52, Michael Paquier ha scritto:
> On Fri, Oct 14, 2016 at 11:27 PM, Albe Laurenz <laurenz.albe@wien.gv.at> wrote:
>> After a successful commit, the WAL file and its metadata are on disk.
>> Moreover, the file metadata won't change (except for the write and access
>> timestamps) because WAL files are created with their full size and never
>> extended, so no WAL file should ever get "lost" because of partial metadata
>> writes.
> This behavior depends as well on the value of wal_sync_method. For
> example with fdatasync the metadata is not flushed. It does not matter
> any for for WAL segments as Albe has already mentioned, but the choice
> here impacts performance.