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

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Message-ID: <20080226170011.GB21203@shareable.org>
Date:	Tue, 26 Feb 2008 17:00:11 +0000
From:	Jamie Lokier <jamie@...reable.org>
To:	Jeff Garzik <jeff@...zik.org>
Cc:	Nick Piggin <nickpiggin@...oo.com.au>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	Chris Wedgwood <cw@...f.org>
Subject: Re: Proposal for "proper" durable fsync() and fdatasync()

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
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