Re: [RFC PATCH v3 04/30] io: fsync before closing a file channel

[Peter Xu <peterx@redhat.com>]

On Mon, Jan 15, 2024 at 09:03:20AM +0000, Daniel P. Berrangé wrote:
> On Mon, Jan 15, 2024 at 04:57:42PM +0800, Peter Xu wrote:
> > On Thu, Jan 11, 2024 at 03:46:02PM -0300, Fabiano Rosas wrote:
> > > > (2) Why metadata doesn't matter (v.s. fsync(), when CONFIG_FDATASYNC=y)?
> > > 
> > > Syncing the inode information is not critical, it's mostly timestamp
> > > information (man inode). And fdatasync makes sure to sync any metadata
> > > that would be relevant for the retrieval of the data.
> > 
> > I forgot to reply to this one in the previous reply..
> > 
> > Timestamps look all fine to be old.  What about file size?  That's also in
> > "man inode" as metadata, but I'm not sure whether data will be fully valid
> > if e.g. size enlarged but not flushed along with the page caches.
> 
> If the size wasn't updated, then syncing of the data would be pointless.
> The man page confirms that size is synced:
> 
> [quote]
>        fdatasync() is similar to fsync(), but does not flush modified metadata
>        unless  that metadata is needed in order to allow a subsequent data re‐
>        trieval to be correctly handled.  For example, changes to  st_atime  or
>        st_mtime  (respectively, time of last access and time of last modifica‐
>        tion; see inode(7)) do not require flushing because they are not neces‐
>        sary for a subsequent data read to be handled correctly.  On the  other
>        hand, a change to the file size (st_size, as made by say ftruncate(2)),
>        would require a metadata flush.
> [/quote]

I should have read more carefully, sorry for the noise.

-- 
Peter Xu