[mythtv-users] question about RAID

[Johnathon Meichtry]

Steve,

All RAID0 does is join (or grow) partitions together.  It is very useful if 
you have, for example, three 100GB disks about like them joined into a 300GB 
partition.

What I have is follows on my Master Backend:

Root Disk (/boot, /,Swap and /cache for ringbuffer ):         Dual 160GB 
SATA disks mirrored (RAID1 using Logical Volume Manager).  /cache has 92GB 
allocated for the ringbuffer.
Media Disk Array (/mnt/store):          Intel SATA 6 Port RAID PCI card with 
4 x 500GB SATA disks in hardware RAID5 array mounted as one 1500GB 
partition.  All rippped DVD's and TV are stored on this partition along with 
a whole bunch of non-Myth stuff such as personal files etc.

Since I have a mix of hardware and software RAID I must say that I don't 
notice a performance benefit between either i.e. they seem to perform about 
the same and are about as reliable as each other.  The benefit to the 
hardware raid card is that it gives me 6SATA ports with no CPU performance 
hit and I keep two ports spare in case I need to grow/add disks or replace 
failures.  The moral is to use whatever suits your situation, budget or 
available hardware - both work just as well as the other.

If I can recommend anything to anyone about disks it is to go down the SATA 
route.  They are hot swappable (i..e even if it doesn't say hot-sweappable 
on the packaging you can plug in a SATA disk while the server is on) and 
secondly get a PCI-Express SATA controller as normal PCI quickly becomes a 
bottleneck on media servers.  I am yet to find a PCI-Express x16 SATA 
controller but once I do I will be buying it as it would be many times 
faster than PCI and finally allow the drives to work at speeds they were 
designed for.  That's just my two cents worth for IDE vs. SCSI vs. PATA vs. 
SATA vs. Hardware RAID vs. Software RAID.

One more thing ... never forget MTBF  (mean time before failure) ... every 
drive has one, and is probably something like 100,000 hours on an old drive 
and the more you use the disk the closer you get to it ...my point is that 
systems like MythTV have the disks working a heck of a lot if not constantly 
therefore getting forever closer to your MTBF so use old disks with a touch 
of caution and be prepared to lose whatever is on it.

Regards,

Johnathon



----- Original Message ----- 
From: "Steve Adeff" <adeffs at gmail.com>
To: <mythtv-users at mythtv.org>
Sent: Monday, January 09, 2006 3:03 PM
Subject: Re: [mythtv-users] question about RAID


> On Sunday 08 January 2006 19:50, Gavin Haslett wrote:
>> RAID 0 would be a straight stripe with no data protection. You were
>> talking about three drives, so in order to use all three you'd be
>> talking this or a RAID 5 in order to get some modicum of protection.
>
> I was under the impression RAID 0 was the fastest RAID, but has no data
> protection? Is there no way to get RAID 0 to use more than one drive? Like 
> I
> said, I'm not concerned with redundency on the RAID0 since I plan to save
> recordings I want to keep to a RAID5 array.
>
>> If you wanted to go RAID 1 or RAID 0+1 then you'd need an even number of
>> drives, and total capacity would be half the actual aggregate capacity
>> of all those drives.
>>
>> Note that in order to create your RAID you're going to have to re-
>> partition your existing disk... so a backup and restore is inevitable.
>
> My plan was to use the two new drives to setup the RAID0, copy the files 
> over
> from my current drive, then remount everything so that the RAID0 becomes 
> my
> recording directory.
>
>> I know you said data protection is not an issue... but believe me when I
>> tell you that it can become an issue rapidly. You can build a
>> significant collection of recordings on any reasonable size media (I
>> have mirrored 160Gb drives myself), and there's no recovery if you lose
>> a single disk. It's not like you can cut the array down to half the
>> array and still get half your data back... you'll lose it all if one of
>> the drives in a RAID 0 fails. RAID 5 and RAID 1/0+1 would at least allow
>> you protection.
>
> Which is why I plan on a RAID5 for my archives drive. I'm hoping HD-DVD 
> makes
> a large presence upon its release and I can just buy the shows I keep in
> HD-DVD and not need to archive them for a long time.
>
>
>> >From a sheer cost perspective I'd say RAID 5 is the way to go... but
>>
>> bear in mind that the processing overhead of RAID 5 plus the data load
>> of reading and writing data and parity in a RAID 5 will make it slow,
>> especially since you run significant risk of saturating the PCI bus with
>> SATA.
>>
>> My dual 160's work great for me right now, and if I wanted to expand I'd
>> just add another pair of drives and continue my RAID 0+1 I set up. I
>> have thought about buying a third and going RAID 5 (would double my
>> storage). Since I'm running ATA133 instead of SATA (drives were cheaper
>> at the time) I am not really stressed about saturating the bus... but my
>> poor old AMD 800 might not be happy with me ;)
>
> -- 
> Steve
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