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

[Jeff Garzik <jeff@garzik.org>]

Jamie Lokier wrote:
> 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.

Oh certainly.  That's why we have a VFS :)  fsync for NFS will look 
quite different, too.


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

Yep.  That would immediately cover a bunch of filesystems.


> 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.

My own idea is that we create a FLUSH command for blkdev request queues, 
to exist alongside READ, WRITE, and the current barrier implementation. 
  Then FLUSH could be passed down through MD or DM.

	Jeff