Hard drive bandwidth - 2 HD Tuners & 4 Analog Tuners

[electronics4lif]

Hello,

I recently upgraded my SageTV server and have a question about hard drive bandwidth. My old server had one hard drive for the OS and SageTV recordings. If I was recording multiple shows at once during prime time, my HD shows would routinely have glitches recorded into them. The glitches were verified to be in the recordings by using various extenders and clients to check the recordings. I believe my hard drive was the bottle neck. I have 4 SD Tuners and 1 HDHomerun (2 HD Tuners).

My question is, how many concurrent SD & HD recordings has everyone else been successful in writing to a single hard drive? My new server has 1 OS hard drive and 2 storage hard drives. I need to know if I need to split the bandwidth up 3 ways or only 2 ways using the forced_video_storage_prefix setting.

Thank you,
Anthony

[GKusnick]

I routinely record three or four HD shows simultaneously to the same drive. Drive bandwidth shouldn't be an issue since each HD stream is on the order of 2 MB/sec (~7 GB/hour), whereas sustained drive transfer rates are in the neighborhood of 30-60 MB/sec.

However if you're writing recordings to your boot drive, then I'm guessing it's formatted in 4K clusters, and that will make a big difference. For best results, you really want a separate recording drive formatted in 64K clusters in order to minimize seeking during recording.

[paulbeers]

A fairly new seperate hard drive should be fine for this. Although bandwidth really won't be the issue, it will be the simultaneous writes that are actually more of a problem. Modern hard drives have a sustained read and write of 60MB/s (or better) and as Gkusnick stated that HD is only around 2-3MB/s and SD is less than 1MB/s. The real issue as stated is that your hard drive will be writing 6 streams of data at one time which can be a bit more taxing, but overall I would imagine you would be fine.

[electronics4lif]

Originally Posted by GKusnick

I routinely record three or four HD shows simultaneously to the same drive. Drive bandwidth shouldn't be an issue since each HD stream is on the order of 2 MB/sec (~7 GB/hour), whereas sustained drive transfer rates are in the neighborhood of 30-60 MB/sec.

However if you're writing recordings to your boot drive, then I'm guessing it's formatted in 4K clusters, and that will make a big difference. For best results, you really want a separate recording drive formatted in 64K clusters in order to minimize seeking during recording.

Thanks for the reply. I had my OS hard drive partitioned so that the recordings were using 64K clusters while the OS was using 4k clusters. Maybe the problem was only occurring due to the fact that I didn't have a separate physical hard drive? Are you using a separate physical hard drive for your recordings where you are able to do 3-4 HD streams?

[electronics4lif]

Originally Posted by paulbeers

A fairly new seperate hard drive should be fine for this. Although bandwidth really won't be the issue, it will be the simultaneous writes that are actually more of a problem. Modern hard drives have a sustained read and write of 60MB/s (or better) and as Gkusnick stated that HD is only around 2-3MB/s and SD is less than 1MB/s. The real issue as stated is that your hard drive will be writing 6 streams of data at one time which can be a bit more taxing, but overall I would imagine you would be fine.

The separate drive may be the key. Either way, this time I will have 2 separate hard drives for recordings. Sounds like I should be golden with only using 2 drives.

[BFisher]

I use 2 recording hard drives for 6 tuners (3 HD, 3 SD) and play via 5 extenders (mix of HD and MVP) and have never had an issue. You should be fine.

[electronics4lif]

Originally Posted by BFisher

I use 2 recording hard drives for 6 tuners (3 HD, 3 SD) and play via 5 extenders (mix of HD and MVP) and have never had an issue. You should be fine.

Excellent. Do you specify which drive each tuner should record to, or do you let Sage decide? If Sage is deciding, it may choose to record 6 shows at once to one hard drive while the other sits idle. On the flip side, if I specify 3 tuners per hard drive, one drive may fill up faster than the other and cause some recordings to be deleted to make space.

Thanks everyone!

[flavius]

Out of the box Sage tries to distribute the recordings evenly, based on utilization. So, if you have two drives that differ vastly in size, like 100 and 500, Sage will record to the larger drive until the space left approaches the space left on the smaller drive.

[wado1971]

Originally Posted by flavius

Out of the box Sage tries to distribute the recordings evenly, based on utilization. So, if you have two drives that differ vastly in size, like 100 and 500, Sage will record to the larger drive until the space left approaches the space left on the smaller drive.

I'm currently adding a 750GB drive to my server which already contains a 200 & 300 HDD for recordings. I'm considering splitting this new 750 into 2 equal partitions to 'balance' the load. Sound reasonable? Does partitioning come with any drawbacks?

[Opus4]

Originally Posted by wado1971

I'm currently adding a 750GB drive to my server which already contains a 200 & 300 HDD for recordings. I'm considering splitting this new 750 into 2 equal partitions to 'balance' the load. Sound reasonable? Does partitioning come with any drawbacks?

If you are thinking of partitioning so that the new drive isn't used for all the recordings, that won't accomplish much because its new partitions are still empty & are going to grab all the next recordings between them and the partitions are still on the same physical drive, so the drive still has to seek to different places when both logical partitions are being written to. I don't see an advantage to partitioning a single physical drive into multiple logical recording drives.

- Andy

[wado1971]

Originally Posted by Opus4

If you are thinking of partitioning so that the new drive isn't used for all the recordings, that won't accomplish much because its new partitions are still empty & are going to grab all the next recordings

Ya - I'll probably balance them out a bit by manually moving some files from the smaller drives. In the end I understand that the 2 partitions from the larger drive will still use the same r/w head but this approach would still reduce my single 750GB drive usage from 100% (in the beginning) down to about 50% and use the 200 & 300 at about 25% each. If I didn't partition and 'balance' then the larger drive would probably see 100% for the next few months. I like the idea of spreading the 'burden' around.

[Opus4]

Originally Posted by wado1971

Ya - I'll probably balance them out a bit by manually moving some files from the smaller drives.

I had debated mentioning moving files in my last post... if you are willing to move files, you could probably still do that with a single partition & end up with approximately the same free space on all drives. (I'm just reluctant to put all the files on a new drive right away & delete them from existing drives because I have had new drives fail fairly soon.)

- Andy

[GKusnick]

Originally Posted by wado1971

If I didn't partition and 'balance' then the larger drive would probably see 100% for the next few months.

Seems to me a better strategy would be to keep the new drive as one large partition and set the diskspace usage rule for that drive to use only 100 GB or whatever. Then as your older drives fill up, transfer recordings to the new drive and increase the allowed usage for that drive to keep the available recording space in balance.

The problem with partitioning is that it increases the amount of seeking the drive has to do. Simultaneous recordings to the same partition can be interleaved in the same area of the disk, minimizing seeks. Simultaneous recordings to different partitions can't be interleaved and force the drive to continually seek back and forth from one partition to the other. You want to avoid that if at all possible.

[wado1971]

Originally Posted by Opus4

I have had new drives fail fairly soon.) - Andy

Good point! I just read a report from google on hard drive MTBF and it clearly showed that a larger % of drives fail within the first 3 months. Once past this critical break in period the failure rate was shown to drop off.
FWIW: Also interesting in the report showed that a drive running too cool was just as likely to fail as an overheated drive. It convinced me that a drive (all components for that matter) is designed for a normal operating temp with limited range in BOTH +/- degrees (obvious i guess). After reading the report I decided NOT to place my new server in the basement which sees too large a temp swing. If I find the link I'll post.

Originally Posted by GKusnick

Seems to me a better strategy would be to keep the new drive as one large partition and set the diskspace usage rule for that drive to use only 100 GB or whatever

Ok - this sounds reasonable and accomplishes what I want without the hassle of moving files around.

Originally Posted by GKusnick

The problem with partitioning is that it increases the amount of seeking the drive has to do. Simultaneous recordings to the same partition can be interleaved in the same area of the disk, minimizing seeks. Simultaneous recordings to different partitions can't be interleaved and force the drive to continually seek back and forth from one partition to the other. You want to avoid that if at all possible.

And here's the negative to my original thought process - which seems to always have a whole in it somewhere. Thanks for pointing this out. I'll be going with the usage rule approach you suggested.

thanks guys - wado

[SpiffyRex]

However if you're writing recordings to your boot drive, then I'm guessing it's formatted in 4K clusters, and that will make a big difference. For best results, you really want a separate recording drive formatted in 64K clusters in order to minimize seeking during recording.

Right.

The problem with partitioning is that it increases the amount of seeking the drive has to do. Simultaneous recordings to the same partition can be interleaved in the same area of the disk, minimizing seeks. Simultaneous recordings to different partitions can't be interleaved and force the drive to continually seek back and forth from one partition to the other. You want to avoid that if at all possible.

Right again.

I spent a lot of time on a similar problem a few months ago. My HTPC had one 500GB drive with a 10GB OS partition (4K) and the rest on a second recording partition (64K). It has two HD and one SD tuners. It's connected to the TV in the family room and to one MVP in another room. It had always worked fine with smooth playback while recording from multiple sources. My plan is to replace the MVP with an HD extender and add a second one to another room, if the HD extenders are ever available again.
To make sure my HTPC *and* home network can handle the new setup, I placed spare computers where the extenders will go and installed SageTV Client trial software to simulate the HD extenders load. Then tested the setup by recording from all tuners at once while playing back from the HTPC and both clients.
It all worked fine... For a while. Just by accident I noticed that when the recording partition gets more than 70% full, some, sometimes all playback will stutter. No problems when the recording filesystem is < 50% full. What the...? So the first thing I did was to defrag the recording filesystem. Not b/s I had high expectations that it will fix the problem but b/s it was the easiest thing to try. No improvement whatsoever, very same behavior with a 70% full filesystem. After analyzing disk usage in the HTPC I noticed that at full load the OS is paging quite a bit. Not trashing but paging more than usual. Ah! Got it, could it be that when the filesystem starts to get full, the disk drive arm has to seek from the very end of the platter to the very beginning, back and forwards b/s of the increased paging? If you look at the drive as linear storage the OS filesystem and paging file are at the beginning of the drive and the recordings, once the recording filesystem starts to get full, are mostly at the end of the drive. To test this theory I got the recording filesystem 80% full and then I put the paging file at the 1/4 end of the recording filesystem. Big improvement! Still a bit of stutter every now an then but now I knew what was the problem. Since I was running with only 1GB of memory, the OS will start to page with all the simultaneous recording and playback. The solution was not just to add another 1GB of memory, which I did, but to put the OS (and paging file) in an old 80GB drive that I wasn't using anymore. Maybe I didn't need more memory at all but since I had to open the HTPC to put the drive I wanted to cover all bases. Smooth playback after that! But still waiting to get two HD extenders.

[blade]

I don't really see why adding a new drive should be an issue. A couple of months ago I added a new drive and every show was recorded to it for probably a week or two. I regularly captured 2 hd shows, 2 sd shows, processed all 4 shows with comskip as they were recording, and streamed to 2 clients and had no issues.

[SpiffyRex]

The issue is partitioning drives, not adding aditional recording drives. If your new drive has one partition with a 64K cluster filesystem, you will not only get more storage space but better recording throuput.

[kevine]

Originally Posted by wado1971

After reading the report I decided NOT to place my new server in the basement which sees too large a temp swing.

I tend to disagree with this statement. I think the basement has a better all year round temp swing and less day to day temp swing. My HTPC is in the basement as well as my main TV viewing (large HDTV). During the winter, I definitely need a little extra heat but during the summer it is nice and cool. Where as, the main floor needs lots of extra heat and lots of extra cooling.

Originally Posted by SpiffyRex

The solution was not just to add another 1GB of memory, which I did, but to put the OS (and paging file) in an old 80GB drive that I wasn't using anymore. Maybe I didn't need more memory at all but since I had to open the HTPC to put the drive I wanted to cover all bases. Smooth playback after that!

I tend to agree with this statement and believe this to be the root of my problems as well. I have 4 drives (500 GB each same exact model). 1st is partitioned to 50 GB for OS. I believe that was a mistake. I will be moving th OS off to a smaller drive shortly.