[mythtv-users] Raid configurations

Terrence Martin tmartin at physics.ucsd.edu
Fri Aug 13 21:54:39 EDT 2004

Shawn wrote:

>Anyone see any performance problems if I used four WD2500JD drives hung
>off a Promise SATA150 TX4 in software RAID6 for video storage? Of
>course, XFS on top of that.
>I just really don't want a single failure on a huge RAID0 video volume
>to destroy ALL my video.
>I figure that'd satisfy my storage cravings for a while, and be
>virtually free of worry for me, barring some form of corruption.
>mythtv-users mailing list
>mythtv-users at mythtv.org

Far from having performance issues you will find performance to be very 
nice indeed.

I run this for my storage array at home. Here is my config.

FC2 + Promise 150TX4 SATA controller + 4x Seagate 160GB SATA Drives + 
PIII 800Mhz + 2.6 Kernel + XFS

I just ran a test.

52MB/s read, 33MB/s write

Definitely not performance to sneeze at for a $60 SATA controller and a 
3 year old computer. Particular on writes since raid5 is not know for 
its write speed. The Linux kernel has a very fast RAID5 implementation.

My tests are done with a simple dd command

#Write test
time dd if=/dev/zero of=bigfile.zero count=1000000 bs=1024
# Read test
time dd if=/dev/bigfile.zero of=/dev/null

Then I do some quick bc math.

I think this is a pretty good test for myth since this is similar to 
what you expect for video, long multi gigabyte reads and writes. My 
tests are 1GB which is approx 3 times RAM so disk cache effects would be 

33MB/s will give you plenty of head room for multiple tuners.

A word of caution. RAID5 is the most disk efficient of the RAID 
personalities. However it is also more risky that a true data mirror. 
There is a non-zero chance that when you go to rebuild your array after 
a disk failure that one of your other disks could fail during that very 
intensive and time consuming process. I have first hand experience with 
this sort of thing on a much more expensive hardware array (4.8TB 
gone...:( ). If you can when a drive fails copy the data onto another 
disk or some other backup. Then rebuild the array with the new drive. 
Better to be safe than sorry.

Also if you have a unclean shutdown the array will take several hours to 
re-initialize itself. During this time you will get reduced performance 
during this initialization phase, although linux throttles this process 
to 10MB/s.  This is different than a fsck (ext2) and file system type 
does not effect it. It is simply a requirement in raid5 on unclean 
shutdown to scan every block and test the parity.

Those caveats aside I have run RAID5 for many years both at work and at 
home and it is a good compromise between data security, cost and 
performance. I have lost 3 disks over the last 3 years but not one bit 
of data.

On the disks, it is less space but if you get 4x200GB seagates ($131 ea 
at newegg.com) you get 5 years of warranty. You may only get 1 with the 
western digital. Also the Seagate are native SATA drives. We have pretty 
much switched to seagate for all of our cluster nodes where possible. 
Too much hassle with WD and Maxtor.


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