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Re: LVM RAID5 with missing disk?

On 05/12/14 05:01 AM, Mart van de Wege wrote:
Gary Dale <extremegroundmailer@gmail.com> writes:

On 04/12/14 12:51 PM, Dan Ritter wrote:
On Thu, Dec 04, 2014 at 02:13:59PM +0100, mad wrote:

I wanted to create a RAID5 with lvm. The basic setup is something like

lvcreate --type raid5 -i 2 -L 1G -n my_lv my_vg

which would mean 3 physical drives would be used in this RAID5. But can
I specify that one drive is missing as it is possible with mdadm?
I don't think so, no. You can create your RAID with mdadm and
put LVM on top of that.

In general I strongly recommend against using RAID5. RAID1, 10,
or 6 are all better options if your data's availability is
important to you.


Sorry, but there are good reasons to use RAID 5 and better reasons to
NOT use RAID 10. RAID 1 and RAID 5 are both immune to single disk
failures in their most common configurations (1 or more data disks
with 1 parity disk).
The problem is not that RAID5 does not provide resilience against a
single disk failure. The problem is that with modern disk capacities,
the chances of *another* disk failing while the array is rebuilding have
significantly risen.

Especially when all the disks came out of the the same batch, they tend
to fail at similar times. I know Best Practice is to mix disks in RAID
arrays, but who actually practices that, instead of just taking the risk
of failure and covering it with a higher RAID level, like RAID6 in this

The chances of two disks failing within hours of one another is very small even for disks from the same batch. RAID 6 is preferable when you have large arrays as each additional disk raises the odds. It's also good when the location is remote so it may take a while to replace a failed disk. But for more typical arrays RAID 5 is good enough considering that RAID 6 uses more power, costs more and has reduced performance compared with RAID 5.

Again, this is talking about software RAID. With high-performance hardware RAID controllers, RAID 6 is often as fast as RAID 5.

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