[zfs-discuss] zfs-discuss Digest, Vol 43, Issue 3

Gionatan Danti g.danti at assyoma.it
Wed Nov 7 13:42:31 EST 2018


Il 07-11-2018 15:48 George Melikov via zfs-discuss ha scritto:
> Cherry on a pie:
> - consumer HDDs: Non-recoverable read errors per bits read = <1 in
> 10^14, so manufacturers are nearly guarantee a one bit failure on
> ~11.37TB of data read

This is not given at all. First, a 1 in 10^14 URE rate means that each 
read has 1/10^14 chances to fail which, for 10^14 bit read (100 Tb or 
12.5 TB), roughly translate in (1-(1/10^14))^(10^14) == 0,368% to 
*successfully* complete all reads.

But the key point is that manufacturers do *not* explicitly states as 
the URE rate is estimated, or what conditions it precisely measured. 
 From here [1]: "It is important to stress that there is no generally 
agreed upon interpretation of bit error rates."

For example, are UREs due to bad writing, so that a subsequent read 
fails?
Are they due to degradation of the underlying magnetical media?
Can an URE "disappear" if trying to read the sectors to a later time 
and/or with different environment conditions (ie: temperature)?
As HDDs are block devices, does the URE rate accounts for the underlying 
block size?
Etc...

The details *really* made a world of difference here. I read over 50 TB 
from a 500 GB disk with a 1<10^14 URE rate with absolutely no problem. 
But this does really means nothing...

An interesting (but absolutely not conclusive) dicussion on the matter 
can be found here [2]

[1] 
http://citeseerx.ist.psu.edu/viewdoc/download;jsessionid=BAADCF565B20D3947B209A374889352D?doi=10.1.1.41.3889&rep=rep1&type=pdf

[2] https://www.spinics.net/lists/raid/msg46845.html

> - enterprise HDDs: <1 in 10^15, so a one bit failure on ~113TB of data 
> read
> 
> https://en.wikipedia.org/wiki/RAID#URE
> 
> 07.11.2018, 17:36, "Maurice Volaski via zfs-discuss"
> <zfs-discuss at list.zfsonlinux.org>:
>>>  IMO, disks much over 4TB are bad news. I suggest you stick with 4TB 
>>> disks.
>>>  Avoid shingled or helium filled disks, or those under 7200rpm. 
>>> Consult last
>>>  few years worth of Backblaze statistics when choosing the disk brand 
>>> and
>>>  model to get.
>> 
>> I believe no answer has been given as to why one should avoid > 4TB 
>> drives (or helium-filled disks for that matter). Interestingly, the 
>> latest backblaze stats recommend helium-based models.
>> 
>> _______________________________________________
>> zfs-discuss mailing list
>> zfs-discuss at list.zfsonlinux.org
>> http://list.zfsonlinux.org/cgi-bin/mailman/listinfo/zfs-discuss
> 
> ____________________________________
> Sincerely,
> George Melikov
> 
> _______________________________________________
> zfs-discuss mailing list
> zfs-discuss at list.zfsonlinux.org
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Danti Gionatan
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