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Re: LVM RAID5 with missing disk?

Gary Dale a écrit :
> On 05/12/14 03:35 PM, Pascal Hambourg wrote:
> You can think of the RAID algorithms as parity checks. A mirror is even 
> parity.

This point of view is a bit twisted, but I can understand and won't argue.

> While the disks are not physically assigned to be data or 
> parity, you can recreate a failed RAID 5 disk by recalculating the 
> parity based on the surviving disks.

Not only the parity but also the missing data.

>> Linux can use a special RAID 10 mode (mirror+stripe) with two or three
>> disks.
> If you have different sized disks, yes.

Why ? Linux RAID, as most RAID types, uses disks of the same size.

> The more usual case is to use 
> similar disks. If one disk is not striped, you lose some of the 
> performance improvement. RAID 10 with two disks makes little sense.

Linux RAID 10 allows striping even on two disks ("far" mode).

>>> with 6 disks, RAID 6 will give you double the capacity of 4 disks
>>> or get you immunity to 3 disks failing.
>> RAID 6 can survive 2 disk failures regarless of the number of disks in
>> the array.
> You misread the sentence. You can run RAID with any number of parity 
> disks by tweaking the algorithms. Most people don't bother using more 
> than 2 parity disks but there is no theoretical reason why you couldn't, 
> to get as much safety as desired.

Misread ? You mentionned RAID6. RAID 6 is a standard RAID level
consisting of block-level striping with double distributed parity,
providing fault tolerance up to 2 failed drives. Not 3. Of course you
can define any algorithm you like, but if your custom RAID has triple
parity, then it's not RAID 6.

> Prior to being able to boot to an 
> mdadm RAID 5 array, I regularly had 3 to 5 disk RAID 1 /boot partitions 
> - why not use the disks since they are already there and it keeps the 
> partitioning the same across the drives.

Of course. RAID 1 as a standard level is not limited to 2 mirrored
disks. It can have any number.

> RAID 6 can be considered a tweaked RAID 5 and RAID 5 can be considered a 
> tweaked RAID 1.

Going this way, anything can be considered a tweaked anything else, if
you tweak enough...

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