Question about LVM and RAID

[moi]

RAID5 (if it really is one) ALWAYS has one drive´s capacity as spare... 
the spare blocks are just distributed on the disks, thus avoiding the 
bottleneck of a single spare drive (these would be raid levels 3 and 4).

what you meant was RAID6/ADG, a semi-proprietary stuff rather found on 
hardware controllers, e.g. hp smartarrays. these do calculate a parity 
for each n blocks, and for "n blocks+parity" generate a second parity 
block. All these blocks are distributed evenly on all drives in the array.

The thing with ADG is the rebuild time - for example the RAIDs at work 
have about 20 drives each (300 gig); the rebuild time on those is about 
1 gb per hour minimum (when there is heavy activity on the raid set). 
that would mean 300 hours without any protection (when using raid5) ! 
instead, with raid6/adg there still is one parity left.
bad thing, though, is the raid controller has to calculate a lot of 
parities. furthermore, the cost is rather high with 2 disks´  worth  of 
parity. Most of the time, such setups use RAID10 (mirror and stripe), 
which uses much cheaper controllers and offers more performance.

sorry for off-topic :)

Robin Laing schrieb:
> rainer wrote:
>
>>
>> Kanwar Ranbir Sandhu wrote:
>>> Hi All,
>>>
>>> I have an external drive cage which has been configured with two
>>> separate RAID 5 arrays.  I then used LVM to create two PVs, and then
>>> added the volumes together under one VG.  The whole shebang is mounted
>>> on one file system (/srv).
>>>
>>> What would happen if one of the RAID arrays failed (e.g. two drives die
>>> in RAID 5 array 1)?  Would the data be safe, would I lose all data, or
>>> would I just lose the data that was on the failed array?
>>>
>>> I believe I would only lose the data on the failed array, but a friend
>>> believe I would lose the whole lot.
>>>
>>> Thanks in advance!
>>>
>>> Regards,
>>>
>>> Ranbir
>>>   
>>
> > Hi,
> >
> > I suppose you have only made a JBOD with LVM - no further RAID0 or 
> such.
> >
> > With JBOD, at least the data on the left RAID5-set should be safe. The
> > data on the failed array and the bits and pieces which were on both
> > arrays would be lost. Fragmentation and the size of data could be an
> > issue - e.g. a 200 GB file on 2 x 160GB arrays would mean more than 
> half
> > is lost... ;(
> >
> > I think your friend meant you had made a RAID0 above the RAID5-sets,
> > which would indeed mean the whole thing would be lost.
> >
> > Regards,
> > rainer
> >
>
> To add to this.
>
> A RAID 5 array is dependent on the number of drives where the data is 
> spread across.  If there are 3 drives, then two failures is more than 
> enough.  If you have 6 drives, then two drives may be okay.
>
> It would be more useful to explain the RAID setup with the hardware, 
> number of drives per RAID and how they are configured.
>
>