Unfortunately there is a 'rebuild hole' in many redundant configurations.
In RAID1 that is when one drive fails and just one remains. This can be
eliminated by running 3 drives so that 1 drive can fail and 2 would still be
operational.
There are plenty of charts online to give % of redundancy for regular RAID
arrays.
With a modern filesystem capable of multiple copies of each file this can be
overcome. ZFS can handle multiple drive failures by selecting the number of
redundant copies of each file to store on different physical volumes.
Simply put, a ZFS RAIDZ with 4 drives can be set to have 3 copies which
would allow 2 drives to fail. This is comewhat better than RAID1 and RAID5
both because more storage is available yet still allows up to 2 drives to
fail before leaving a rebuild hole where the storage is vulnerable to a
single drive failure during a rebuild or resilver.
Standard RAID is not going to have this capability and is going to require
more drives to improve though each drive also decreases reliability has more
drives are likely to fail.
ZFS also is able to put metadata on a different volume and even have a cache
on a different volume which can spread out the chance of a loss. very
complicated schemes can be developed to minimize data loss.
This is precisely the need for next-gen filesystems like ZFS and soon
BTRFS. To fill these gaps in storage needs. Imagine the 10TB drives of
tomorrow that are only capable of being read at 100MB/s. Thats a 30 hour
rebuild under ideal conditions. even when SATA3 or SATA6 are standardized
(or SAS) you can cut that to 7.5 or 15 hours but that is still a very large
window for a rebuild. On-line rebuilds and filesystems aware of the disk
systems are becoming more and more relevant.
On Mon, Jun 1, 2009 at 5:03 PM, Adam Goryachev <
[email protected]> wrote:
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> Tom Brown wrote:
> > -----Original Message----- From: Adam Goryachev
> > [mailto:[email protected]] 2) Use 2 hard drives
> > of equal size in a RAID1 mirror (or 3 or more in a RAID5 or RAID6)
> > to ensure that your backups will survive a disk failure.
> >
> > RAID1 or RAID5 will certainly reduce the likelihood of data loss,
> > but I'm not sure that either RAID configuration ensures data
> > protection. I've read arguments that, as the size of drives
> > increase, the likelihood of RAID5 protecting data decreases. An
> > offsite backup of your data is your best insurance.
> >
> > This can be a contentious issue, and I am no expert. Here are
> > links.
> >
> > http://blogs.zdnet.com/storage/?p=162&tag=nl.e539
> > http://miracleas.com/BAARF/RAID5_versus_RAID10.txt
> >
> > As Adam said, just my opinion. And probably less informed than his.
> >
> While we all agree that RAID5 is better than none, how much redundancy
> is ever enough? Even an off-site backup has to be stored on some
> media... is that RAID5/6/10/etc ...
>
> Do we need to use RAID61 (Would that be a group of RAID1 arrays in a
> RAID6) or is that still not enough? Can we afford to put that many
> drives into a single system (ATAoE/iSCSI?)?
>
> So, what do people out there do for their live data, as well as their
> backuppc data? What happens if you lose all the data on your backuppc
> system, is that considered a catastrophic loss?
>
> Curious to see some discussion... hopefully we can avoid the flame
> wars....
>
> PS, from the article, it seems to me that even RAID1 is eventually
> going to have the same issue, because when you try to rebuild, you
> only ever have one source of data (unless you use 3 disk RAID1)....
>
> Regards,
> Adam
>
> - --
> Adam Goryachev
> Website Managers
> Ph: +61 2 8304 0000 [email protected]
> Fax: +61 2 8304 0001 www.websitemanagers.com.au
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>
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