Re: parity data

[Chris Mason <chris.mason@oracle.com>]

On Mon, 2008-09-08 at 18:46 -0600, Eric Anopolsky wrote:
> On Mon, 2008-09-08 at 10:47 -0400, Chris Mason wrote:
> > On Sat, 2008-09-06 at 23:43 -0600, Eric Anopolsky wrote:
> > > Hi,
> > > 
> > > A couple of questions.
> > > 
> > > 1. Does btrfs currently have anything like raid 5 or 6?
> > > 
> > 
> > Not yet, it might one day.
> > 
> > > 2. One guy on my LUG's mailing list is really excited about the
> > > potential for setting redundancy on a per-file basis.
> > > I.e. /home/eric/criticalfile gets mirrored across all of the drives in
> > > the filesystem but /home/eric/temporaryfile gets striped. I'm skeptical.
> > > Is it a good idea to allow people/programs to do this?
> > 
> > In general, yes.  Some files or directories are crucial, and some (swap
> > for example) don't need to survive a crash.
> 
> If a disk dies in a redundant configuration, I'd like to be able to hot
> replace the failed disk and keep going without any interruption. So
> losing parts of the paging file would be pretty bad in that case.
> 
> Isn't a partially failed array (where some files are accessible and
> others are not, without any additional filesystem damage) a weird
> failure mode? Do people know how to deal with this? Do applications know
> how to deal with this?
> 
These configurations are not new.  Admins create different filesystems
on different storage all the time.  From an admin point of view, one
file on thier box isn't accessible and they want to carry on.

The fact that it is one file in a single FS or one file among dozens of
filesystems doesn't change things.

> What kind of file would be important enough to keep around but
> unimportant enough that it could be lost at any time while the system is
> up without anyone knowing or caring?
> 
> > But, I think the flexibility should go a little further.  The goal is to
> > be able to define drive groups and tie files or directory trees to the
> > drive groups.  That way you can say these files go to the fastest drives
> > and these files go to some other drive type, etc etc.
> 
> Let's say I have 4 100GB drives (2 fast ones and 2 slow ones). I've
> restricted a performance critical directory to the two fastest drives,
> currently totaling 100GB of performance critical data. The rest of the
> data on the system is striped.
> 
> How much free space do I have on the filesystem? 100GB (the amount of
> data I can store in the performance critical directory)? 200GB (the
> amount of data I can store outside the performance critical directory if
> the striping is guaranteed)? 300GB (the amount of data I can store
> outside the performance critical directory if the striping is best
> effort)?
> 

People already create these configurations, they just do it with
multiple filesystems.  And, when they want to resize the performance
critical section, it is a difficult (and often slow) operation.

More flexibility in managing storage is the end goal for btrfs, and
we're just barely getting to the point where we can start addressing
these difficult issues.

-chris