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:
Kanwar Ranbir Sandhu wrote:
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!
> I suppose you have only made a JBOD with LVM - no further RAID0 or
> 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
> 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.
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.
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