[mythtv-users] Slightly OT - How many People have Video libraries over 8TB?

[Another Sillyname]

On 7 July 2012 22:32, Raymond Wagner <raymond at wagnerrp.com> wrote:
> On 7/7/2012 13:34, digid myth wrote:
>>
>> I would never build a raid with 2 TB drives. Takes too long to rebuild
>> the raid and 60% of the time you will have a 2nd drive fail during the
>> rebuild.
>
>
> Huh? There was a whole lot of FUD several years ago about UBE rates making
> it probable you experience a second bit failure by the time you finish a
> recovery. So a brief lesson in statistics and hard drive design, what is
> going on here?
>
> A UBE (unrecoverable bit error) or URE (unrecoverable read error) is simply
> when a hard drive fails to decode a single bit. You don't actually read the
> data directly. That requires some really expensive gear, and is too slow.
> You read most of a whole bunch of redundant information, and decode your
> data back out of it using CRC recovery codes. Many hard drives are rated for
> a UBE rate or less than one in 10^14 bits. That means within 100Tbits, or
> 12.5Tbytes (or 11.37TB), you are less than 50% likely to experience an event
> in which you don't read enough information off the platter to decode your
> data from that sector.
>
> So what happens when you experience a UBE on a RAID array? These things are
> expected due to the design of hard drives. It doesn't indicate a failed
> drive. It merely indicates a single failed read. The RAID array detects the
> fault, decodes the data from the parity block, writes it back to the drive
> the fault occurred on, and life goes on. Some companies sell "RAID Edition"
> drives, and the only difference between these and the standard consumer fare
> is that the standard drive will go to Herculean lengths trying to re-read
> that sector to recover the data, while the RAID Edition drive will fault out
> early, allowing the array to recover the data on its own, rather than stall
> operations for an extended period.
>
> What if it's a RAID0 array, or critical RAID5 array? As mentioned, this
> isn't a drive failure, it's only a sector fault. The RAID controller isn't
> going to expel the card and render itself incomplete. That would be stupid.
> It's going to warn the administrator, and may wait for confirmation, before
> going ahead and completing the recovery of the rest of the array. Before
> with a single drive, you just lost the sector, 512B or 4KB, while now the
> worst case is you lose that whole stripe, maybe 1MB or so. Of course you
> could always run RAID6, where after the loss of that first drive, you still
> have a second parity set, and your chances of two UBEs occurring within the
> same stripe on two drives is infinitesimally small.
>
> There is no RAIDocalypse. We are not all going to lose our data. We will
> just get to the point where an array with no redundancy left guarantees some
> loss of data.
>
>
>> In the corporate world no drives larger than 500G or so are typicly put
>> in a raid group for just this reason.
>
>
> In the corporate world, no drives larger than 500G or so are typically put
> in a RAID group, for the sole reason that they simply don't exist. They just
> don't make 10K and 15K SAS drives larger than 600GB.
>
>
>> Everytime I have lost something on a raid it was during a raid rebuild
>> of a failed disk.
>
>
> In all likelihood, you bought a bunch of hard drives at the same time, from
> the same vendor, from the same manufacturer, such that they were all built
> in the same production batch. You then proceeded to operate them in the same
> temperature conditions, under the same use profile. It is not at all
> unreasonable that you would experience two of them failing at nearly the
> same time. That's why you're supposed to buy drives from separate lots, and
> periodically cycle them, so you don't get a bunch ready to fail
> simultaneously.
>
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I have banking clients with Arrays using 2TB+ drives, the largest
array I've seen was about 100TB usable IIRC consisting of 75 2TB
drives in RAID6 with hotspares, this was a production array for data
archiving so not mission critical but a legal requirement, running
24x7 and kept in a very cool data centre (ambient air temp is about
12c), there's at least 800 servers in that centre so the AC costs
probably run £1m+ PA.