Re: [opensuse] bigger disks, bigger risks?

On Wed, Nov 12, 2008 at 11:53 AM, Per Jessen <per@xxxxxxxxxxxx> wrote:
Richard wrote:

So, it is no longer theoretical; you need RAID both on the primary
AND backup hardware devices. That gives you the best fault
tolerance, both human and hardware.

What do you do about the risk of a dual-drive failure? RAID6 is one
possible answer, but AFAIK it requires at least 5 disks, which is too
many (for my situation).


Per, you can always toss more and more drives making the RAID more
and more bulletproof, but simultaneous multiple drive failures
fortunately are very rare.

The key problem I see is that whilst the risk has always been very low,
it is slowly increasing due to the enormous amounts of space per drive.

A SOHO should be able to take the risk that multiple drive failure
will not occur in any given 24 hour (or so) period.

Yep, that sounds entirely reasonable.

A large corporation or one like banks, etc, where *any* loss is
potentially catestrophic, multiple machine continuous backups
including at least one off-site machine needs to be implemented, all
with RAID 6 protection. This is the kind of backup that 9-11 at the
WTC used and while everything in the buildings was lost including many
computers with sensitive data lost, the off-site and transactional
backups running continuously allowed for little or no loss of data for
the datasets so protected.

There is a very wide gap between your SOHO above and the DR situation of
a large corporation, with dual datacentres and all that. In between
there are many smaller businesses who can easily afford to take care of
the dual drive failure risk whilst they can't afford to protect
themselves against a 747 landing on their datacentre :-)

In my opinion Per, a 4 drive RAID 5 is exposed to 'degraded' operation
very infrequently,

Very true, but disk-space is so cheap that it's worth looking into. One
reason I'm looking into 3-drive RAID1 is the write-performance penalty
of RAID5/6.

and when it is, the odds of a 2nd drive failing are almost
microscopic, so at a hardware level, a 3 or 4 drive RAID 5 is
an acceptible risk. Again, if your data is valuable enough, invest
in the extra drive(s) and use RAID 6 or even RAID 6 cascaded with RAID
1 'protection' of the entire array....what is that? Raid 60?

Too many drives involved - we are limited to 4 drives per system.


--
/Per Jessen, Zürich

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Hi, all

I am missing one aspect of this discussion, one I learned about not to
long ago:
Large hdd's have a bigger chance at a bad block. The maximum accepted
bad block rate is expressed in % of blocks, but as there are more
blocks on a disk there is a bigger chance at a bad block.

I do not have in depth knowledge of this, but I read (in the same
article, can't find it now) a rebuild operation cannot continue once
it has encountered a bad block.

Is this true?

Neil

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a future to work to.
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