I have recently updated my hardware and now ready to purchase new drives to create a new pool with more storage.
Whilst researching different drive options I have decided to go for a 5 disk z2 pool with 3TB hdd's as it seems to be cost effective and within budget.
My current z1 pool is 3x2TB WD Red.
I have stumbled across a few articles that discuss URE and the chance of disk failure when re-silvering. Based on the details provided by WD my current drives have a value of <1 in 10^14
whereas the WD Red Pro & WD Gold have the value <1 in 10^15
This may not seem too important but look at a thread in another forum it could make a significant difference :(https://forum.synology.com/enu/viewtopic.php?t=96171)
It seems it may be much safer to go for the more expensive Enterprise drives, but with 5xhdd's this then changes the total cost significantly. Also I'm struggling to find 3TB Hdd's in the pro variant or another manufacturer with <1 in 10^15 values.
What are your thoughts?
Whilst researching different drive options I have decided to go for a 5 disk z2 pool with 3TB hdd's as it seems to be cost effective and within budget.
My current z1 pool is 3x2TB WD Red.
I have stumbled across a few articles that discuss URE and the chance of disk failure when re-silvering. Based on the details provided by WD my current drives have a value of <1 in 10^14
whereas the WD Red Pro & WD Gold have the value <1 in 10^15
This may not seem too important but look at a thread in another forum it could make a significant difference :(https://forum.synology.com/enu/viewtopic.php?t=96171)
Hard drives have built-in logic that is intended to correct certain types of read errors - the errors that escape without automatic correction are classified as unrecoverable read errors (URE). If you take a look at the manufacturer's specifications, you will see that the specifications list less than one in 10^n bits for the URE statistic:
Western Digital Red
http://www.wdc.com/wdproducts/library/S ... 771442.pdf
<1 in 10^14
Western Digital Red Pro
http://www.wdc.com/wdproducts/library/S ... 800002.pdf
<1 in 10^14 (it seems like this document a couple of months ago used to state <10 in 10^15)
Western Digital RE4
http://www.wdc.com/wdproducts/library/S ... 701338.pdf
<1 in 10^15
So, that <1 in 10^14 URE statistic could mean that one URE might be encountered in every 10^16 bits read. You might want to look at that statistic more as a measure of how well the drive manages bit rots (that is the eventual decay of magnetism that controls whether a bit is read as a 0 or a 1, not necessarily the chance of encountering an error when reading the same bit 10^n times).
Let's go back to the formula that I provided in my initial reply:
(1 - (99,999,999,999,999 / 100,000,000,000,000) ^ 48,000,000,000,000) = 0.380979164
So, for this RAID 5 array, rebuilding the array (when a failed drive is replaced) has a 38.1% probability of failure.
What about if enterprise class drives are used, with a 1 URE in 10^15 bits read statistic:
(1 - (999,999,999,999,999 / 1,000,000,000,000,000) ^ 48,000,000,000,000) = 0.046829645
Note that the probability of failure is now 4.7% - a considerable improvement.
The formula for a RAID 10 array results in the number 0.015860094, so that is roughly 1.6% probability of failure during the rebuild of the RAID 10 array.
It seems it may be much safer to go for the more expensive Enterprise drives, but with 5xhdd's this then changes the total cost significantly. Also I'm struggling to find 3TB Hdd's in the pro variant or another manufacturer with <1 in 10^15 values.
What are your thoughts?