[mythtv-users] .23/.24 NFS mounting / Storage Groups

[Steven Adeff]

On Wed, Oct 20, 2010 at 7:57 PM, Brent Bolin <brent.bolin at gmail.com> wrote:
> On Wed, Oct 20, 2010 at 6:44 PM, Raymond Wagner <raymond at wagnerrp.com> wrote:
>> On 10/20/2010 19:36, Brian Wood wrote:
>>>
>>> On Wednesday, October 20, 2010 05:23:41 pm Scott wrote:
>>>>
>>>> On Oct 20, 2010, at 5:22 PM, Brent Bolin wrote:
>>>>>
>>>>> My current setup is a back end with raid disk storage and tuners.  I
>>>>> also have a back end running in a Xen environment as a guest helping
>>>>> with commercial flagging.
>>>>>
>>>>> I also just upgraded my network to GB today.
>>>>>
>>>>> Flagging from the mythtv guest helps take the cpu load off the primary
>>>>> back end, but still puts a stress on disk I/O.
>>>>>
>>>>> I've been toying with the idea of creating another storage directory
>>>>> for mythtv recordings and that being an NFS mount from the primary
>>>>> back end.  One of my main concerns is creating a complected
>>>>> configuration.
>>>>>
>>>>> Is this the only way to spread disk usage between systems(NFS)?
>>>>
>>>> Are the disks really stressed during commflag? I have a single disk SATA
>>>> 7200 RPM drive that records 3 HD streams from a HDHR while doing commflag
>>>> at the same time. Disk IO has never been a bottleneck on the system.
>>>>
>>>> If you do want to spread the work, your best bet for simplicity is more
>>>> physical drives in the same system. You can use mdtools or LVM to strip
>>>> data across two disks. to spread the work.
>>>
>>> Linear RAID or RAID 0 will "spread the load" and increase apparent disk
>>> performance, but also increase the probability of losing the array's
>>> contents due to a disk failure, as those systems provide no redundancy.
>>
>> RAID0 is actually a detriment to recording performance.  If given
>> independent drives, MythTV will default to balancing the load evenly across
>> multiple drives.  Each drive will only have to seek between the recordings
>> it is directly handling.  When doing RAID0,  every disk will be dealing with
>> every recording, seeking back and forth between each, and your total
>> throughput diminishes accordingly.
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>
> The raid setup is mdadm(md) raid5 with LVM on top(5x1GB sata).  CPU
> load(md) is never more then 2%.  Not really interested in any other
> raid configurations.
>
> Same type of raid setup on the Xen box.  I do not want to put tuner
> cards in the Xen box.
>
> The setup is 6 HD PCI(e) tuners.  So often in prime time I have 6 HD
> recordings running and 4 instances of flagging.
>
> So getting back to my original question about spreading the I/O
> between systems.  NFS is the only option correct?.  Unlike the xen
> guest that streams from the back end for flagging.  Think I read also
> different methods of load balancing.

to answer your question (again), no you can use samba to share
"drives" across computers, but NFS is the best option for this type of
thing, especially since it will preserve file information like owned
users and such.

I used to have a 6 drive RAID5 array with 300GB drives as my recording
drive. This started well before storage groups and LVM were much the
rage for MythTV. Having 2 QAM and 2 Firewire tuners I was able to
easily overload it if recording 4+ streams and trying to commflag. It
was becoming an "issue" and so about 6mo ago or so I decided to use my
6 media server drives (500GB, faster as well) and to go with the
storage group model, since I didn't need the RAID 5 redundancy. Best
decision I made, where before I would get up to 80% io wait times in
top at full tilt, now I've seen times where my system is recording 6
streams and running 4 commflag processes and my io wait fluctuates
between ~2% and 20%, I never notice any dropouts, etc like I used to.

BIG thumbs up to going with storage groups.


-- 
Steve
http://www.mythtv.org/wiki/User:Steveadeff
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