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What protection does XRaid offer for large disk arrays?
I’ve been running my 314 for 3 years now with a mixture of disks Disk 1 – Seagate 2TB (ST32000542AS 24k+ hours) Disk 2 Seagate 3TB (ST3000DM001 18k+ hours, the one that has high failure rates) ...
Retired_Member wrote:
Instead, in your situation I would either go with
1) 4 WD Red 3TB or
2) 3 8TB (WD or HGST preferred)
As has been pointed out, XRAID will let you use 2x8TB+3TB with no issue.
Just to comment on the economics, using current Amazon US pricing:
WD30EFRX: $110.00
WD80EFZX: $265.00
4x3TB costs $330 (since he is reusing the WD30EFRX he already has), that gives him 6 TB more storage than the existing WD30EFRX has by itself. So that is $55 per TB gained.
2x8TB+3TB costs $530, and gives 8 TB more storage than the existing 3 TB drive. That is $66.25 per TB gained.
So it is the case that 4x3TB is more economical in the short run. But later on, you will pay a much higher price to expand it again.
For example,
- adding 5 TB to the 4x3TB system will cost $530 (upgrading two drives to WD80EFZX). That is $106 per TB gained.
- adding 8 TB to the 2x8TB+3TB system costs only $265 (adding a single WD80EFRX) That's only $33 per TB gained.
hp532 wrote:
I’ve been reading up on Raid5 and wonder what protection XRaid (which I understand to be Raid5 + volume expansion) offers when used with large capacity disks? Most places suggest that given disks have an error rate of 1 in 10e14 you’d expect a 11TB array rebuild after a disk failure to fail with around 90% probability (11/12.5).
Those are the "death of RAID" sites I think.
But lots of people here have single-redundancy arrays that are bigger than 12 TB, and they have resynced them successfully (often many times). Every time you do a scrub you are reading all the sectors, and I do that every three months on my 15 TB, 16 TB, and 18 TB RAID-5 volumes with no problems. The math says that just won't work.
So it's clear that you can't just blindly apply the math. The error rate is spec'd at < 10e14, and the "less than" clearly is important. Drives are much more reliable than that spec suggests.
Retired_Member wrote:
Instead, in your situation I would either go with
1) 4 WD Red 3TB or
2) 3 8TB (WD or HGST preferred)
As has been pointed out, XRAID will let you use 2x8TB+3TB with no issue.
Just to comment on the economics, using current Amazon US pricing:
WD30EFRX: $110.00
WD80EFZX: $265.00
4x3TB costs $330 (since he is reusing the WD30EFRX he already has), that gives him 6 TB more storage than the existing WD30EFRX has by itself. So that is $55 per TB gained.
2x8TB+3TB costs $530, and gives 8 TB more storage than the existing 3 TB drive. That is $66.25 per TB gained.
So it is the case that 4x3TB is more economical in the short run. But later on, you will pay a much higher price to expand it again.
For example,
- adding 5 TB to the 4x3TB system will cost $530 (upgrading two drives to WD80EFZX). That is $106 per TB gained.
- adding 8 TB to the 2x8TB+3TB system costs only $265 (adding a single WD80EFRX) That's only $33 per TB gained.
hp532 wrote:
I’ve been reading up on Raid5 and wonder what protection XRaid (which I understand to be Raid5 + volume expansion) offers when used with large capacity disks? Most places suggest that given disks have an error rate of 1 in 10e14 you’d expect a 11TB array rebuild after a disk failure to fail with around 90% probability (11/12.5).
Those are the "death of RAID" sites I think.
But lots of people here have single-redundancy arrays that are bigger than 12 TB, and they have resynced them successfully (often many times). Every time you do a scrub you are reading all the sectors, and I do that every three months on my 15 TB, 16 TB, and 18 TB RAID-5 volumes with no problems. The math says that just won't work.
So it's clear that you can't just blindly apply the math. The error rate is spec'd at < 10e14, and the "less than" clearly is important. Drives are much more reliable than that spec suggests.
I am a heavy believer in dual redundancy especially as they come out with ever larger disks.
Consider these factors;
when you rebuild/resync a raid 5 device, you are putting a heavy load on all of your drives.
the larger the array is, the longer it takes for a rebuild to finish
so you are putting a heavy load, for a longer period of time, the bigger your array is
if your array is rebuilding because of a failed disk, and your other disks have similar hours on them, you are now putting a heavy load, for a long period of time, on disks that have a lot of hours on them already.
with the 6 and 8 bay devices, raid6/dual redundancy is the default, but if you started out with only a few disks you might still be in raid 5/single redundancy mode.
Either way, with only a single redunancy, if anything bad happens during a rebuild, you are likely screwed plain and simple. this forum and every other nas forum are littered with the tears of those who assumed that raid 5 would protect them and did not have a separate backup of their data.
neither RAID or a NAS is backup by itself. neither will protect you from theft, fire, flood, or malicious users.
a backup means a completely separate copy of your data, ideally multiple copies, in multiple different places.
raid6/dual redundancy will better help keep your data safer during rebuilds/resyncs, if a drive fails during a raid 6 rebuild, you will still have your data intact.
Of course if you have really bad luck and multiple drives fail, again you are likely screwed without a separate backup.
in summary, neither raid, nor nas is a replacement for backups, backups, and more backups.
RAID modes aside, you can't extrapolate RAID breakdown from the URE specs - 1 in 10e14 is clearly very conservative. I suspect that's to motivate people to buy enterprise-class drives. But that doesn't mean that multiple drive failures don't occur - they do, and lots of people here have been hurt by them.
TeknoJnky wrote:
I am a heavy believer in dual redundancy especially as they come out with ever larger disks.
That's a reasonable approach, as long as people don't use that as a subsitute for backup (which of course you don't).
I use single redundancy myself, but I maintain two backups on other ReadyNAS (the three volumes I mentioned above are my primary and two backup NAS).
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