2 disks can be critical for RAID-10 of the 4-disks. Those. A single disk failure will never break the data. If you are interested in the RAID-10 option (software), in which
The failure of two disks is guaranteed not to violate the data, then how many of them in RAID
put Just in on-line calculators (different) I get at 8 disks in RAID-10
(on any quantities in the group) that only one broken disk will never taske the data.
How to consider the number of disks in RAID-10 correctly to save the data when you exit two, or three disks?

Answer 1, Authority 100%

RAID-10 in standard duplication mode without losing data will survive the loss of one of any disk, the resistance of the second loss is not guaranteed. Assembled as a mirror over full, or on the contrary, full of mirror, or MDADM (which is neither one or the other) – no difference is guaranteed stability without any single disk. Special attention to “any disk”.

Each data block in RAID-10 is mirrored on two disks, because of this, the total loss of tank is half. But therefore, if you are not lucky, and even those two, on which there were mirrors of one data sector, have fallen out even 10 discs – this sector read more nowhere. In the event of a great good luck of the falling discs, it is possible to lose up to half of the drives of the array.

For example, in Linux RAID Aka MDADM, it is possible to specify how many copies of the data should be replicated by disks. For example, 3 copies of data on 6 disks will give you the opportunity to survive the loss of any two disks and not any 4. The price of this is the available capacity of the array. You will be available for the capacity of only two disks from 6.

RAID5 and RAID6, which you write in the comments – will survive the fallout of one and two disks, respectively. The failure of any second disk in RAID5 or any third in RAID6 is fatane and the loss of the entire array. The goal and the lot of these raid levels are to progress from the death of the disk, but at the same time somehow more cheap than the mirror. RAID5 will reduce the formatted array capacity to the size of only one disk, RAID6 is on the capacity of only two disks. And not half, like RAID1 or RAID10.

For example, from 12 disks of 1 TB can be collected:

• Raid5 with a capacity of 11 TB, you can lose any 1 disk
• RAID6 with a capacity of 10 TB, you can lose any 2 disk
• Raid10 with a capacity of 6 TB, you can lose any 1 disk
• Raid10 with a capacity of 4 TB, if you configure that you can lose any 2 disk

It would seem why then is the RAID10 so actively use with such a difference in the capacity? Answer: Due to performance. RAID10 The read request can serve any disk from the pair, it means that the RAID10-made RAID10 can be swapped by different disks. RAID5 / 6, one source data block is stored only in one place. To read it from redundant data – it will be necessary to read this segment from all disks immediately and apply a bit of mathematics. Then, RAID5 / 6 is permanent on the record. And much more dramatic difference in degraded form, i.e. If one drive fell from us. RAID5 / 6 deals with performance more than sensitive.

How many disks can be lost – solves the task. Let me remind you that when the drop-down disk is replaced with a new one and the process of synchronization of the array begins – this is a very dangerous time, on old discs the load increases sharply and someone else can die. Therefore, RAID5 is quite rare, RAID6 is not much more expensive for these tasks, but protects the Masiva rebeling time.

And another important point, which is necessary when talking about raids, always indicate: RAID is not backup. Backup You should have anyway.