Re: Proposal for "proper" durable fsync() and fdatasync()

[Jamie Lokier <jamie@shareable.org>]

Jeff Garzik wrote:
> Nick Piggin wrote:
> >Anyway, the idea of making fsync/fdatasync etc. safe by default is
> >a good idea IMO, and is a bad bug that we don't do that :(
> 
> Agreed...  it's also disappointing that [unless I'm mistaken] you have 
> to hack each filesystem to support barriers.
> 
> It seems far easier to make sync_blkdev() Do The Right Thing, and 
> magically make all filesystems data-safe.

Well, you need ordered metadata writes, barriers _and_ flushes with
some filesystems.

Merely writing all the data pages than issuing a drive cache flush
won't Do The Right Thing with those filesystems - someone already
mentioned Btrfs, where it won't.

But I agree that your suggestion would make a superb default, for
filesystems which don't provide their own function.

It's not optimal even then.

  Devices: On a software RAID, you ideally don't want to issue flushes
  to all drives if your database did a 1 block commit entry.  (But they
  probably use O_DIRECT anyway, changing the rules again).  But all that
  can be optimised in generic VFS code eventually.  It doesn't need
  filesystem assistance in most cases.

  Apps: don't always want a full flush; sometimes a barrier would do.

-- Jamie