I currently run 3x 2TB Samsung F3EG (1 URE in 10^15) in a software (mdadm) RAID5 in my main computer. I'm running out of free space & want to build a separate NAS running FreeNAS, using these 3x 2TB disks & 3x new 2TB disks - which will be 1 URE in 10^14, because when Seagate bought Samsung's disk division & ceased production of the 1 in 10^15 F4EG disks there are no longer any 10^15 disks that aren't 'enterprise' models with the accompanying price tag.
I want to use ZFS, either RAID-Z2 or RAID-Z3, using all 6x 2TB disks, however I'm having difficulty working out the probability of multiple UREs when rebuilding. In a worst-case scenario the array will be full, so when a single disk fails it will need to read all of the remaining 5x disks to rebuild, which means 10TB of data. It's fairly easy to work out that the probability of a single URE when reading 10TB & assuming all disks are 1 URE in 10^14 is given by;
100/(10^14)*8.8E13 = 88%
Which isn't great at all, so RAID-Z with a single layer of parity is out of the question. However I don't know how to calculate the probability of getting a *second* & even a *third* URE in that same 10TB of read data. Can anybody with better maths knowledge than me help?
Also wrt UREs, can anybody confirm what happens when ZFS encounters a URE that it can't recover from (eg there is a failed disk on a RAID-Z2 & then 2x URE's when rebuilding). I've read in some places that ZFS will alter the filename of any affected files so that they are identified as 'unrecoverable' & then continue rebuilding. But I've also read that when a disk produces a URE the controller/ZFS marks the entire disk as failed & the rebuild would stop/fail.
As for hardware, I'm planning on using the Fractal Array R2. I've read that a dedicated controller doesn't really provide any performance benefit when using ZFS than software, so I'm planning on getting a Mini-ITX motherboard with 6x SATA connectors. The only choice that is still stocked seems to be the Zotac H67ITX-C-E, which I would pair with an i3 & 8GiB or 16GiB RAM. I'm guessing that H67 is fairly well supported & there shouldn't be any issues with the onboard LAN?
For the disks, I already have the 3x 2TB Samsung F3EG & will probably go for Western Digital for the new 3x. I don't trust Seagate's quality as much as WD & now that Samsung have been taken over by Seagate they're one & the same. My first choice would have been the 2TB WD Green, but I've read that the 'Advanced Format' feature that doesn't use standard 512B sectors makes using them with FreeNAS more difficult. The WD Red is the next choice, especially as they claim to be '24/7' disks, but I can't tell if they also have this 'Advanced Format' rubbish. Any input?
For RAM, will I benefit from having 16GiB instead of 8GiB? The NAS will normally only be serving a single computer, if it makes a difference. The price of 16GiB is exactly double that of 8GiB, so I don't mind getting 16GiB if I will notice a difference.
tl:dr
1.) How to calculate probability of multiple UREs in a certain amount of data read?
2.) Is this a good selection of hardware?
Fractal Array R2
Zotac H67ITX-C-E
i3 2120
8GiB or 16GiB RAM
3x 2TB Samsung F3EG
3x 2TB WD Red (WD20EFRX?)
I want to use ZFS, either RAID-Z2 or RAID-Z3, using all 6x 2TB disks, however I'm having difficulty working out the probability of multiple UREs when rebuilding. In a worst-case scenario the array will be full, so when a single disk fails it will need to read all of the remaining 5x disks to rebuild, which means 10TB of data. It's fairly easy to work out that the probability of a single URE when reading 10TB & assuming all disks are 1 URE in 10^14 is given by;
100/(10^14)*8.8E13 = 88%
Which isn't great at all, so RAID-Z with a single layer of parity is out of the question. However I don't know how to calculate the probability of getting a *second* & even a *third* URE in that same 10TB of read data. Can anybody with better maths knowledge than me help?
Also wrt UREs, can anybody confirm what happens when ZFS encounters a URE that it can't recover from (eg there is a failed disk on a RAID-Z2 & then 2x URE's when rebuilding). I've read in some places that ZFS will alter the filename of any affected files so that they are identified as 'unrecoverable' & then continue rebuilding. But I've also read that when a disk produces a URE the controller/ZFS marks the entire disk as failed & the rebuild would stop/fail.
As for hardware, I'm planning on using the Fractal Array R2. I've read that a dedicated controller doesn't really provide any performance benefit when using ZFS than software, so I'm planning on getting a Mini-ITX motherboard with 6x SATA connectors. The only choice that is still stocked seems to be the Zotac H67ITX-C-E, which I would pair with an i3 & 8GiB or 16GiB RAM. I'm guessing that H67 is fairly well supported & there shouldn't be any issues with the onboard LAN?
For the disks, I already have the 3x 2TB Samsung F3EG & will probably go for Western Digital for the new 3x. I don't trust Seagate's quality as much as WD & now that Samsung have been taken over by Seagate they're one & the same. My first choice would have been the 2TB WD Green, but I've read that the 'Advanced Format' feature that doesn't use standard 512B sectors makes using them with FreeNAS more difficult. The WD Red is the next choice, especially as they claim to be '24/7' disks, but I can't tell if they also have this 'Advanced Format' rubbish. Any input?
For RAM, will I benefit from having 16GiB instead of 8GiB? The NAS will normally only be serving a single computer, if it makes a difference. The price of 16GiB is exactly double that of 8GiB, so I don't mind getting 16GiB if I will notice a difference.
tl:dr
1.) How to calculate probability of multiple UREs in a certain amount of data read?
2.) Is this a good selection of hardware?
Fractal Array R2
Zotac H67ITX-C-E
i3 2120
8GiB or 16GiB RAM
3x 2TB Samsung F3EG
3x 2TB WD Red (WD20EFRX?)