[mythtv-users] question about RAID

Steve Adeff adeffs at gmail.com
Mon Jan 9 20:16:25 UTC 2006


On Monday 09 January 2006 14:32, Johnathon Meichtry wrote:
> ----- 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
> >
> >
> 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.

which is roughly what I would like to do (well, maybe not 500GB quite yet). 
I'll have to see if I can get more than 2 disks for the RAID1 array, if not, 
hopefully 2 will be fast enough for what I need.

I notice you are using LVM for your RAID1, why did you choose this over mdadm?

> 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.

=) already going down this route, all my new drives are SATA. I'm also going 
to wait for a PCI-Express x16 SATA controller before I bother buying one, 
same reasons as you.


> 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

-- 
Steve


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