[mythtv-users] OT: LVM, partitioning over multiple drives

[Michael Miyabara-McCaskey]

RAID5, is simple math "x + y = z", therefore the min is 3 drives... But
you lose 1 disk in the process... so 3 disks = lose 1/3rd... 4 disks =
lose 1/4th... etc... the max num of disks depends on the HW controller, or
for Software RAID on linux I dont believe there is a limit.

Obviously your usable space, gets better with the more drives you
have... Also RAID5 has very good read performance with larger block sizes,
which should do very well with large files... like video! yeah!

but....

RAID5 write performance sucks, because it takes a fair amount of CPU to
calculate the "parity" (redundancy info) bits, during a write operation.
(which is why most servers doing RAID5 have their own CPU on the HW raid
card + memory to buffer the write operations).

Now if you have a dedicated FileServer, that isn't also your Myth Backend,
then it may be fine using Software RAID... and it would be the cheapest
way to get a large volume + redundancy.

-Michael


On Thu, 13 Jan 2005, Les Gondor wrote:

> At the cost of the aggregate space not being the sum of the capacity of
> the individual drives, the only way to get complete fault tolerance is
> to make the volume a RAID-1 or RAID-5 configuration. Since you are
> already at capacity on at least one spindle, I'm guessing that mirroring
> will cost too much in terms of capacity to be viable for you. I don't
> know what the minimum number of spindles are for LVM RAID-5, but for N
> drives of size S, the capacity will be (N-1)*S. I would count on at
> least 4 drives being needed for RAID-5.
>
> Cheers,
> Les Gondor
>
> Michael Knoll wrote:
> > My video collection has expanded over multiple harddrives, and
> > currently I'm using a symlink farm to unify the collection.  I was
> > looking into LVM to join the drives into one partition, but I am
> > concerned about the risk of failure.  If I join the two drives into
> > one partition, I've added the failure risk.  Now either drive has to
> > fail to lose all my data, rather then one drive fail to lose half my
> > data.
>