W dniu 08.07.2020 o 00:40, Grant Taylor pisze:
On 7/7/20 8:52 AM, R.S. wrote:
Few words about RAID:
RAID is more reliable than single disk. Assuming same reliablity of
disk used in RAID.
That starts to get questionable when you have more and more disks in a
RAID array.
That's why you DON'T HAVE more and more disks in RAID group. Note: dasd
box is not single array. Usually it is group of arrays.
Not to mention specific arrays made by Moshe Ianai, like XIV. Those
arrays are more reliable with more disks.
Typical dasd boxes use RAID5 4+1 or 7+1, so more disks mean more such
groups, not more disks in the group.
And again: single drive is less reliable than RAID5 composed from such
disks.
Note: a system consisting of N elements and probability of each element
failiure is 1/N. Failure of any element cause system failure. Let's say
N=1000000. Many people thinks such system will fail for sure. It is not
true. For large values on N the probability of working system strives
toward 1/e.
It's a numbers game of how likely is it to have two drives fail in a
RAID 5 or three drives fail in a RAID 6.
I think somewhere around twenty drives and you're back down to, or
below, the reliability of a single drive.
See above.
[snipped]
--
Radoslaw Skorupka
Lodz, Poland
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