[mythtv-users] Question re: available SATA ports and linux software RAID

digid myth digidmyth at gmail.com
Sat Apr 9 13:42:40 UTC 2011

Here is my take on this.

My goal was to get the power footprint down on my main server in the house
which is also my Myth Backend. We have 2 Myth Front Ends, and 2 Crystal
Cinema Media Players for the Kids rooms. My goal is to run an total average
of 100 watts on this server. I am not there yet, with numbers running around
120-150 watts each month. This is down from 400 watts for the old server.
For a living I am a system administrator for some large scale systems so am
very familiar with availability and data recovery.

Here are a couple of rules that I have for my installation.

All backups are to other disks, I have discontinued use of tape.
A data drive or raid set can not share the same power supply as its backup
drive. I have had a PS go bad and take out all the drives connected in the
Backup drives should be kept spun down or left off unless in use.
Keep the drives as cool as possible, and in standby if possible.
Drive Access speed only needs to be slightly faster than the max network
transfer rate.
    (I am very happy with the 2TB Samsung Spinpoint F4 drives)
No drive will have active data on it until it has been on line and running
for 4 weeks.
All drives are replaced every 3 years, old drives may be used for
offline/offsite backup storage.
The more drives in a raid array the more likely you are to have a failure,
so if you do not need high availability, I try to stay away from it on this
Video archives do not change much and are not high availability so they do
not live on a raid.
Backup Everything that is important or takes a long to to recover.

My main server is a dual core AMD with 6 Gig of ram. Video source is a HD
Homerun on an antenna.
It runs the Myth Backend, Zone Minder Security System, with 4 cameras, Anti
Spam for all our mail accounts, A OSX VM that keeps the family's calendars
and music in sync, and occasionally transmission and handbrake. I am on the
edge of using all the resources of this server at times, but it is not
uncommon for it to be sending 3 streams out, and recording 2 channels at the
same time, and ripping a DVD and everythings works fine.

Attached to the server is a rosewell RSV-8 storage tower that holds 8
drives. I purchased from NewEgg

This tower supports 8 drives on 2 sata channels and uses sata port
multiplication, which means that is shares the bandwidth of 4 drives with
the bandwidth of 1 sata channel. This has not proven to be a problem in my
setup.  It comes with a 2 channel PCIe ESATA card, and cables. I had
problems with the cables and replaced them with more flexible higher quality
ones, and have not had any more problems with the drives going off line.

In the main dual core server.

  2 DVDROM Drives
  2 320G sata boot drives in raid 1 (These are very fast seagate drives)
  2 2TB sata drives in raid 1
  1 2TB sata drives that we use for our OTA recordings from myth
  2 2TB sata drives in raid 1 that we use for general data storage

  In the 8 bay tower
  2 2TB sata drives for backups from the laptops and workstation on the
  2 2TB sata drives that hold our general movie collection set up to myth as
storage groups.
  2 2TB sata drives that are spares / general purpose drives.
  I have 2 bays open for future expansion.

  I have a 2nd server that was built for speed and on demand uses. It has a
4 bay sata enclosure that holds drives for backups from the main server.
It holds
 2 2TB drives to backup the movie collection to.
 1 2TB drive to backup the data raid to
 1 1TB drive to hold the main server backups.

  This server is normally powered down.  This server is up only when I need
its resources. I use it to run VM's and to rip DVD's etc. The drives are
only up when backups are run. (Just a note this servers power is not in my
measurements that I mentioned above, as it is an on-demand server, and was
build for speed and not power savings)

  The one hole in my system is that the OTA recording are not backed up or
raided at this time. My next plan is to move this to a raid, but it will
still not be back backed up. The myth database lives on the main raid and is
backed up to the data raid. Storage groups join the main movie drives, so
the size of the drives does not matter, therefore it does not need to be
raided, and I can add drives as needed with out having to reconfigure the
filesystems etc.  As one drive fills up it changes very little, so backups
for full drives are uncommon. On the drive we are adding movies to, I do a
manual backup to the 2nd server when I add movies to it. All other backups
are automated, and I use custom software that I wrote that works very much
to Apples Time Machine, using rsync. Laptops and workstations are backed up
every day, the server drives are backed up every week, or before and after I
make massive changes. This makes backups very quick and painless as only
changed data is backed up. Data that has not changed is simply linked to the
file in the previous backup.

To help save on drive wear and tear I turn off the recording of access times
(noatime in the fstab file) to off on all drives but the OS drives, and the
Myth OTA recording drive.

 All drives are put in sleep mode after they have been idle for 30 min. This
works well on the software raid1 but I have had problems doing this with
software raid5 and raid6. If you are monitoring the drives using the SMART
utilities there is a flag you can set to check to see if the drives are
asleep and it will skip them. Otherwise SMART will wake the drive up, or
never let it go to sleep.

Remember you have to take the whole system into account, and one big raid in
my opinion is definitely not the way to go with myth. The developers have
given us storage groups for a reason. The advantage of storage groups is
that adding disk space only requires you to add the drive, no changes need
to be made to your existing data, which is always safer. Only the active
drive needs to be awake and running when you are recording, or streaming a
movie, This reduces the overall time on the drive, and reduces the heat that
is generated. Heat is the number one killer of hard drives. If you loose a
drive you only loose the data on that drive, not a whole array. I have seen
many raid5 and raid6 arrays fail either through hardware or software. If
your controller goes bad the whole array is lost in some cases.  It is not
uncommon for  someone make a mistake rebuilding a failed array and wipe the
whole array out. Never rely on RAID as your backup. With 2TB drives commonly
sold for $75.00 at newegg the price of the backup drive is peanuts. Your
backup plan also needs to include some time of offsite backup for you most
critical data. My 2TB raid on the main server is copied out to and external
2TB drives and taken off site at least twice a year. I have 2 drive I rotate
out off site for this. Digital Photo's, personal data etc are kept on this
raid so everything that is really important is off site incase of fire,
flood, theft etc.

This setup has worked good, and has evolved over the years into what it is
today. Getting the power down is more of a hobby than a necessity. About the
only way I am going to be able to get the average any lower is to eliminate
the external drive tower and put all the drives in the case, with a high
efficiency power supply. I am considering it, but the external case sure is
nice, and I am not sure that it is worth the 20 watts for so that I could
save. Remember that the biggest way to improve the reliability of your
system to to eliminate as much heat as possible. If you can keep the
server(s) in a basement, and keep everything cool you will get good service
from your system.

I hope this will give you some ideas, and also help those that are building
or evolving their MythTV servers.

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