On Wed, Nov 12, 2008 at 11:53 AM, Per Jessen
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 -- While working towards the future one should be ensuring that there is a future to work to. ----------------------------------------------------------------------- ** Hi! I'm a signature virus! Copy me into your signature, please! ** ----------------------------------------------------------------------- -- To unsubscribe, e-mail: opensuse+unsubscribe@opensuse.org For additional commands, e-mail: opensuse+help@opensuse.org