I have been looking into RAID5 Vs RAID6 lately and I keep seeing that RAID5 is not secure enough anymore because of the URE ratings and increasing size of the drives. Basically, most of the content I found says that in RAID5, in case you have a disk failure, if the rest of your array is 12TB, then you have almost 100% chance to meet a URE and to lose your data.
The 12TB figure comes from the fact that disks are rated at 10^14 bits read to reach one URE.
Well, there is something I do not get here. A read is done by the head going on the sector, what can make the reading failed is either the head dies or the sector dies. it can also be that the reading does not work for some other reason (I don't know, like a vibration made the head jumps...). so, let me address all 3 situations :
- the reading does not work : that is not unrecoverable, right? it can be tried again.
- the head dies : this would for sure be unrecoverable, but, that also means the full platter (or at least the side) would be unreadable, it would be more alarming, no?
- the sector dies : as well totally unrecoverable, but here I do not understand why the 4TB disk is rated at 10^14 for the URE and the 8TB is as well rated at 10^14 for the URE, that would mean the sectors on the 8TB (most likely newer tech) are half as reliable as the ones on the 4TB, that does not make sense.
As you see, from the 3 failure points I identify, none makes sense. So what exactly is an URE, I mean concretely?
Is there somebody who can explain that to me?
After first wave of answers, it seems the reason is the sector failing. Good thing is that firmware, RAID controller and OS + filesystem have procedure in place to early detect that and reallocate sectors.
Well, I now know what is a URE (actually, the name is quite self-explanatory :) ).
I am still puzzled by the underlying causes and mostly the stable rating they give.
Some attributed the failing sector to external sources (cosmic waves), I am then surprised that the URE rate is then based on the reading count and not on the age, the cosmic waves should indeed impact more an older disk simply because it has been exposed more, I think this is more of a fantasy though I might be wrong.
Now comes the other reason that relates to the wear of the disk and some pointed out that higher densities give weaker magnetic domains, that totally makes sense and I would follow the explanation. But As it is nicely explained here, the newer disks different sizes are obtained mostly by putting more or less of the same platter (and then same density) in the HDD chassis. The sectors are the same and all should have the very same reliability, so bigger disks should then have a higher rating than smaller disks, the sectors being read less, this is not the case, Why? That would though explain why the newer disks with newer tech get no better rating than the old ones, simply because the better tech gain is offseted by the loss due to higher density.