Re: [PLUG] Spanning volumes with LVM (Ubuntu)

[Matt Mossholder <matt@mossholder.com>]

There are several issues with utilizing RAID5 for MythTV storage. The biggest is that when you write data that doesn't fill an entire stripe, you end up having to read the stripe in first, so that you can then recalculate the parity of the new stripe.Â

The ins and outs of myth and RAID are documented here.Âhttp://www.mythtv.org/wiki/RAID

These are the key statements, though:

 Â --Matt

On Sat, Sep 28, 2013 at 6:06 PM, Rich Freeman <r-plug@thefreemanclan.net> wrote:

On Sat, Sep 28, 2013 at 5:32 PM, Matt Mossholder <matt@mossholder.com> wrote:
> I would recommend you used the 1TB drive for the OS and DB, and use the
> other two drives for storing recordings. If you want to put other static
> content (pictures, music, etc.) on the recordings drives, that is probably
> OK, as long as you aren't hammering the disks while you are recording.

Honestly, unless you have 10 tuners all running in HD I think this is overkill.

I was running a raid5 across 5 drives containing LVM volumes for OS
and mythtv and was able to sustain recording 4 HD streams at once (on
FIOS, which has fairly high bitrates), along with playback and even
transcoding. ÂGranted, if I did all that and did something
disk-intensive on the side things would bog down. ÂNow, this system
had 8GB of RAM, and I had the write cache set to the max 30s, so I'm
sure that helped a bit.

Since then I have moved my OS to a separate drive, but only so that it
can be on an SSD (with hourly rsyncs to an array). ÂHaving the OS on
an SSD certainly confers benefits.

Note that I am talking about multiple drives on a RAID5, which
obviously has benefits when it comes to large sequential read/writes.
LVM across multiple drives really doesn't buy you much except space
unless you're micro-managing your layouts to try to even out accesses.

Rich
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