I completely disagree with your notion that RAID 5 isn't acceptable. I routinely get 110MB/sec reads and 60MB/sec writes. Copying a file on the same volume is usually in the 45MB/sec range. You don't really say what your hardware is, but I'm doing this with (4) SATA drives and 256MB of RAM cache. Small burst writes to the cache are 290MB/sec. I've recorded (2) SD and (2) HD simultaneously while watching one of those HD streams. Not a single stutter. Might want to check out the LVM_on_RAID page if you ever head back to RAID 5. --DirkGecko 16:30, 13 February 2008 (UTC)
Only 45Mb/s? Doesn't just one HD stream at 1080p use 15 to 25Mbps? Are you using a dedicated RAID 5 card? I was talking about the software solution that comes with Linux.
My hardware was using a mixture of IDE and SATA drives, so that might have been it. It wasn't continuous stuttering that was the problem, it was an intermittent loss of packets during high loads whose only affect on playback was to skip a few seconds. The problem was really intermittent and seemed worse during transitions, i.e. when playback/commercial detection was ending, etc. It might be due to a bug in MythTV too, where database writes, etc. can cause interruption of writing to the disks. TugBoat 17:22, 13 February 2008 (UTC)
I have no idea what one HD stream @ 1080p takes. Depends on the compression format. I do know that when people bandy those numbers about, they're always talking in megabit, not megabyte. The difference being a factor of eight. ATSC is 19.4Mbit/sec (that's ~2.5MB/sec) and QAM is 38.8Mbit/sec (4.85MB/sec). A PVR-*, at it's max bitrate, is going to write 750KB/sec. A decent hard drive shouldn't have much issue doing this. In my case, I am using the good 'ol Linux RAID (with LVM on top of it, no less). And that 45MB/sec I quoted was reading and writing at the same time on the same volume (ex., "cp 8gbfile 8gbfile2"). I have scads of useful disk I/O. Now, I also run a IDE RAID 1 for my system drives, and the performance on those is more-or-less craptastic in comparison, especially on any sort of random access. Is it the IDE vs SATA? I dunno. You might want to also consider changing the I/O elevator you're using in the kernel. Depending on your OS, it could be anticipatory, CFQ, or deadline. I believe in my case, I'm running deadline. Don't quote me on that, though.
Anyway, I'm rambling. My main concern is that your article got listed on the wiki front page (kudos, BTW). It therefor seems to carry some extra weight to the people who read it and it seems to state that RAID 5 is unsuitable. Kinda FUDish is all.--DirkGecko 15:23, 22 February 2008 (UTC)
Yes, you are right, it is 45 MB/s and during my next upgrade, I will again try RAID 5. I have altered the article to point out your notes here. My article made it onto the front page because I put it there. I figured there was no point in writing the article if it was buried on my user page. TugBoat 15:47, 22 February 2008 (UTC)
BTW - I have been using RAID 5 in three MythTV backends for a couple years now and it has worked very well, saving the recordings on more than one occasion when an HD died. TugBoat 22:11, 30 August 2011 (UTC)
Current status on Shaw and 5C?
The Edmonton customer base is being forced to move to HD soon, I'm wondering if I'm going to have to use an HDPVR or if it's worth trying a firewire setup.
My suggestion is to try Firewire first as doing so means the cost of a cable (and time). If firewire works, is stable, and you aren't missing any channels you like, then keep it, otherwise, buy the HDPVR. Setting up the firewire is a good test anyway, because, even if you use the HDPVR, you can use the Firewire 6200ch script that comes with MythTV to change channels and is easier to setup/maintain than using LIRC. We recently reduced the number of channels we get and I haven't been keeping track of which channels are 5C flagged and which are not.
On a related note, the HDPVR freezes once every couple of months and the USB ID of the newer HDPVR box I have isn't in the HDPVR driver yet which creates a hassle when upgrading to new kernels. Of the two HD tuners in the house, the second one still uses firewire and has been hassle free although it does miss recordings once in a while (zero byte) because it can't get a lock. I have a cron job that resets the firewire port every hour and that seems to keep things running. TugBoat 22:11, 30 August 2011 (UTC)