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Re: question on new feature complexity/possibility/sensibility? (^ Alter

To: Linda Walsh <xfs@xxxxxxxxx>
Subject: Re: question on new feature complexity/possibility/sensibility? (^ Alternate)
From: Dave Chinner <david@xxxxxxxxxxxxx>
Date: Mon, 27 Jun 2011 16:02:40 +1000
Cc: xfs-oss <xfs@xxxxxxxxxxx>
In-reply-to: <4E080249.9060903@xxxxxxxxx>
References: <4E067D81.4070605@xxxxxxxxx> <20110627003209.GD32466@dastard> <4E080249.9060903@xxxxxxxxx>
User-agent: Mutt/1.5.20 (2009-06-14)
On Sun, Jun 26, 2011 at 09:08:41PM -0700, Linda Walsh wrote:
> 
> 
> Dave Chinner wrote:
> >So use a filesystem that supports them natively ;)
> ---
>       Got any in in mind that also support acls, extended attrs
> and has the reliability and performance of xfs?  ;-)

For most "normal" workloads, ZFS would probably be your only
production ready option. But that's not something you could use on
Linux, is it? :/

Until btrfs is a completely baked cake, you won't be able to tick
all those boxes, and even then there will be questions about
performance...

Mmmm, cake.... :)

> >>I.e. being able to hardlink files, but have an option to mark it as
> >>copy on write -- allowing space to be save when copying directory trees,
> >>but then dynamically making new copies when someone updates one of the
> >>linked copies.
> >
> >The problem is that a reflink sort of looks like a hard link, but in
> >many cases behaves like a soft link (e.g. different owners,
> >permissions, etc are possible) and hence - combined with the
> >copy-on-write behaviour - they need to be treated more like a
> >soft-link in terms of implementation.
> ----
>       I can see this -- now this doesn't mean we are talking the same
> type of reflink with the shared data above is it?  Cuz in this lower
> case, I was talking about hmmm....interesting....
> 
>       I was thinking of just a copy-of-the whole file on write,
> but that'd be a potential pain on some systems depending on the file
> size....

If you are going to take the pain of copying the entire file anyway,
just copy it up front. XFS is designed for fast, large storage
subsystems, so make use of it's capabilities ;)

> >Soft links have their own
> >inode so can hold state separate to the inode they are pointing to,
> >and for reflinked files it is simply not practical to retroactively
> >modify the directory structure to point at a different inode when
> >the first COW operation occurs.
> ----
>       I see....
> >
> >Like I said, it can be done, but it's not a small project. If you
> >want to sink a significant amount of development time to the
> >project, we will help you in any way we can. However, I don't think
> >anyone has the time to do something like this for you....
> ---
>       If I had the time and mental resources...I'd love to.
> But am a bit overwhelmed for time now, and even if I wasn't, I'd
> not be anywhere near certain I'd be able to maintain continuous focus
> for the length of time necessary to do that length of project....
> if you know what I mean...
> 
>       Maybe certain if it was something that I or others could break
> down into useful 'subchunks' that could go in at separate times.  Nothing
> useless by itself (if nothing less than specific 'upgrading' of data
> infrastructure (data fields on disk, routines to parse such...etc),
> but the whole thing at once seems pretty large.

Actually, none of it needs to be merged until complete support is
available. If there's a need for parts of the work in mainline
before the bigger set of work is complete, then we'd merge the bits
needed and rebase the working tree on top the new mainline tree. git
makes this sort of operation easy, so developement of such a feature
could be done quite simply in a parallel branch.

IOWs, even if I was doing this, I'd still be doing it a small chunk
at a time before committing it to a public branch somewhere. It's
easy to point interested parties at the code that way to collaborate
on a separate branch until everything is done. I just wouldn't be
asking for review/mainline inclusion until I had everything done and
tested....

Cheers,

Dave.
-- 
Dave Chinner
david@xxxxxxxxxxxxx

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