On 10/10/05, Dan Lund <situationalawareness@gmail.com> wrote:

The RAID subsystem takes care of all of the backend issues like
latency to disk and so forth.
nbd is merely a block device, and from my experience if a disk even
has a single timeout, the disk will be written as faulty by the
subsystem and be set to [_].
raid6 is: http://www.synetic.net/Tech-Support/Education/RAID6.htm
Essentially it's data and parity striped across the array, and that
itself has a parity.

You can lose multiple disks with a raid6 setup and not lose data, I
use it and I absolutely love it. (mine is raid6+1)

On 10/10/05, Joseph Sinclair < plug-discuss@stcaz.net > wrote:
> two questions:
> 1) How does nbd deal with the differential latency issue?  If latency differs by too much a RAID system will end up with stripes on different "disks" out of order, and things get REALLY messed up at that point.
> 2) What is RAID 6?
>
> Dan Lund wrote:
> > I've done work like this with the network block device as an
> > experiment in several different ways.
> > To put it in a nutshell I had a machine exporting a couple of nbd
> > (network block devices), and I accepted them on another.  They showed
> > up as /dev/nbd/0, /dev/nbd/1, etc.
> > I then made a raidtab that took them and set them into a RAID5 and had
> > a hotspare.
> > I've tested it with RAID1/5/5+1/6/6+1, made it failover, hot-added
> > "drives", etc.
> >
> > It was pretty decent in throughput, and I was about ready to put
> > together a turnkey solution for my work as an expandable disk
> > subsystem. (on it's own gig backplane)  I made sure it was on it's own
> > gig backplane because the nbd devices are solely dependant on the
> > network.  If it so much as blips, your disks go away.
> > RAID, as far as I know, only works on block devices.
> > You could always check out PVFS, or Coda if your looking for something
> > on the filesystem layer.  I have far more faith in nbd though.......
> >
> > --Dan
> >
> >
> > On 10/9/05, Matt Alexander <lowbassman@gmail.com > wrote:
> >
> >>I'm wondering if anyone knows if this is possible...
> >>
> >> Take multiple remote filesystems such as NFS, gmailfs, Samba, sshfs, and
> >>layer a filesystem over the top to create one namespace.  Ideally it would
> >>provide some fault tolerance/redundancy and improved performance by using
> >>the concept of RAID over the multiple connections.
> >>
> >> In reality, this new filesystem layer wouldn't care if the filesystems are
> >>remote or not.  You could have...
> >>
> >> /mynfsmount
> >> /mygmailfsmount
> >> /myothergmailfsmount
> >> /mysshfsmount
> >>
> >> ...and then a new mount point of...
> >>
> >> /myreallycoolmount
> >>
> >> ...and when you put files here, they're striped/mirrored over all the
> >>previous mounts.
> >>
> >> Is this currently possible?  If not, then perhaps I'll see if I can make it
> >>happen in my minuscule free time.  I know there are a ton of potential
> >>problems with this, but it'd be a fun project nonetheless.
> >> Thanks,
> >> ~M