TiVo Mega - coming soon !!

Discussion in 'TiVo Roamio DVRs' started by HoustonMidtown, Sep 8, 2014.

  1. nooneuknow

    nooneuknow Well-Known Dismembered Member (Lurk Mode On)

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    It's my understanding, that the most likely failure cause for another drive, beyond the first to fail, is the extra load on all the other drives, necessary to rebuilt the one that failed, to a replacement, or a hot spare, drive.

    It's also my understanding that if not for the added workload to rebuild, another drive failure was not necessarily "imminent", under the usual working load, but became so due to the change in workload.

    I'll go out on a limb, and posit that there's really no way to know that one of the remaining drives would fail under the extra load. This is the reason why many say "RAID is not a backup system/solution". I'll make a WAG, that using drives rated for a much heavier workload, than you normally require, when not factoring rebuild load, could lessen, but not eliminate, the threat. That kind of throws the "inexpensive" drive component out of the "I" in RAID. I would think it would be wise to buy drives based on peak "normal" load, plus peak "rebuilding" load, plus room for more (not just "enough").

    I'm only "book smart" on this. So, I don't claim to know everything, or know that what I think to be the best approach, actually is, or will even be enough.
     
  2. tivohaydon

    tivohaydon New Member

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    While it's rare to lose two drives at the same time have you ever stuck around and timed a RAID rebuild on a 4TB drive? It's not pretty. It takes a very very long time on a completely unloaded array! Totally conceivable to me to lose multiple drives while that's going on.

    Could be RAID6. With that much capacity that's what I'd (and do) use.
     
  3. mattack

    mattack Well-Known Member

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    Wait, current Tivos don't have multiple drives/partitions.. if you have a dual drive Tivo, you lose all recordings.. Are you claiming this Mega does essentially have two separate show stores?
     
  4. GmanTiVo

    GmanTiVo give me more options

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    +1 :up:

    my 2 teen daughters & wife are driving me crazy and I am getting tired of waddding throught their crap :(.
    Yeah I know I can get another Roamio, just trying to avoid forking out $700

    Gman
    (Roamio Pro + 3 Minis)
     
  5. flashedbios

    flashedbios Member

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    If this thing had more tuners, and tivo software supported "same channel tuner sharing across multiple minis", then I could see this being used in a small hotel environment or apartment environment, IE, the landlord buys this and a bunch of minis, puts this in the closet, gives the minis to tenants
     
  6. flashedbios

    flashedbios Member

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    also, while rebuilding a drive in an array, tivo's drive contents are constantly changing. something else to cnsider
     
  7. Dan203

    Dan203 Super Moderator Staff Member TCF Club

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    A quick calculation to show why this much space is completely unnecessary... The average bitrate of an HD station is somewhere between 12-15Mbps, which means this thing would hold anywhere from 3600-4500 hours of HD. So you'd need to record 10-12 hours of content every single day, and not delete any of it, to fill this up in a year. Even recording 24 hours a day on all 6 tuners it would take roughly a month to fill this up.
     
  8. flar

    flar Member

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    I'm not claiming anything, just pointing out how that decision might affect the loss of data during a RAID failure. This is all speculation.
     
  9. flar

    flar Member

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    San...
    If all drives are from the same batch then a loss of one drive often correlates with the loss of another, especially if there are 10 drives in play. Some NAS sites recommend getting drives from different batches to reduce the likelihood of a bad batch taking out your entire array.
     
  10. flar

    flar Member

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    San...
    The science of RAID deals with that. The rebuild happens on working drives...
     
  11. flar

    flar Member

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    One reason for only dealing with 6 tuners would be the bandwidth needed to store the output of all 6 tuners (vs 12 or more). Generally with RAID, the write speeds can improve because you split the write requests across all drives, but when you lose a drive, you then have a lot more work to do and so performance drops back quite a bit - especially when you are also trying to rebuild the array when the drive gets replaced. They would have to be able to write 6 streams at once, during "drive failure" mode, while rebuilding the array and while also potentially reading a few streams for the many Minis that they will be pushing for a system like this.

    With RAID 5, for each write operation, you have to read the old data block, the old parity block, modify the parity block to represent the new data block, then write the new data and parity blocks back. The reads and the writes each happen in parallel, though, but what used to be a simple single block write becomes a read and a write and it happens to 2 drives and you are reading and writing the same blocks which means you have to let the platters spin around before you can switch from the reading to the writing. If they are doing 5-drive RAID then they get 5x more ops/second, but 4x the operations (2 reads and 2 writes) so it doesn't end up much faster. If they did 1x10 RAID then the ratio would be better, but the degraded performance would be much worse.

    (It may be better if they did what I conjectured earlier and allocated an entire stripe at a time, then each write operation simply becomes writing 4 data blocks and 1 parity block to all 5 drives simultaneously and no need to read anything back any more, but they transfer 4x the data at once. If standard RAID controllers are already good at noticing sequential writes to all blocks in a slice then they may get this for free.)
     
  12. dswallow

    dswallow Save the ModeratŠ¾r TCF Club

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    Sometimes it's not about recording things one purposefully chooses, but in having useful, somewhat relevant suggestions be recorded and available for view anytime. More space beyond ones needs for purposefully chosen recording means more suggestions remain available to choose from at any given time.

    There's also no need to seek to fill the available space purposefully, but to always have available space should you need it. Going on that 8 week around-the-world cruise? No worries. Your shows will be there to watch anytime.
     
  13. Dan203

    Dan203 Super Moderator Staff Member TCF Club

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    The average bitrate for an HD show is 12-15Mbps. The tested write speed of a WD Green drive is about 100MBps, which means it has enough I/O bandwidth to simultaneously write over 50 HD streams. Now I'm sure the overhead of dealing with multiple streams would reduce that, as would dealing with reads for display and streaming, but supporting 12 streams is not really a stretch. Especially when you've got it RAIDed.

    The 6 tuner limit is most likely a chipset limitation. I don't think the Broadcom chipset they use supports more then 6 tuners. And I'm not sure there is even one in existence that does.
     
  14. aaronwt

    aaronwt UHD Addict

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    It still shouldn't be any problem. When I was testing my RAID5 setup I could still do multiple concurrent transfers to the RAID5 at a couple hundred mb/s when simulating a drive failure. While not near the normal 750mb/s transfer rate it gets, that would still be plenty of speed for six low bitrate broadcast streams.
     
  15. mattack

    mattack Well-Known Member

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    Yes, I did snip your quote.. but I am pretty sure I record this much.. of course I delete stuff after I watch it, but things like late night talk shows I let pile up. Late night talk shows + news + Jeopardy + a bunch of prime time + World News Now.. I think that's easily that much per day.

    (No, I don't WATCH all of the content of all of those.. but for the late night talk shows, I FF through them for the few funny bits/musicians.)
     
  16. nooneuknow

    nooneuknow Well-Known Dismembered Member (Lurk Mode On)

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    Green drives, even the AV-GP, do max out at barely above 100MB/s sequential, (dipping as low as 50MB/s sequential on the inner tracks). Anything not sequential pushes performance off a cliff. But, they are both unsuitable for RAID use, anyway. WD support won't even try to help you, and will tell you to use the right drive for the purpose.

    The WD Red NAS is a green 24x7x365 AV drive, suitable for RAID, and peaks at ~157-160MB/s outer tracks, with up to 120-150TB/yr workload rating, per drive.

    The WD Purple is also green 24x7x365 AV, made for RAID, and use with motion activated surveillance/security cameras, with only a 60TB/yr workload rating, per drive, requiring that the drive have idle time (platters spinning, and minimal activity, as opposed to being saturated). Idle modes and idle time come in multiple flavors. Any time this drive has at least 20% unused workload, is counted as "idle time". WD requires that each drive always have at least 20% of this idle time for the Purple. If it's not getting it, at all times, you are supposed to add drives until you do. So, at least 20% of the time, the drive must also have at least 20% non-saturated workload, or it fails WD's criteria for it being the right drive for the job. This drive is almost entirely impractical, if a market for drives being able to handle a large number of motion-activated cameras, without losing a single frame, did not exist.

    As usual, when I speak of "streams", I'm speaking of the streams as defined by the ATA AV Streaming feature standard, often with increased "streams" supported, by way of proprietary enhancements like SilkStream and AllFrame, which the host must support, or the extra number of streams cannot be used, and will fallback to the base standard.

    Unless TiVo makes a major change, and starts using the ATA AV Streaming feature, the "streams" are limited entirely based on regular data mode read/write performance, and would rely on RAID striping alone, to increase it.
     
  17. aaronwt

    aaronwt UHD Addict

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    Have you tested any of the new 6TB green drives that have been released yet. Their rated transfer rates seem much higher than the lower platter count drives were.

    I have no idea what the green drives are now. I used the WD green 500GB, 750GB, 1TB, 1.5TB, and 2TB drives many years ago and still have some in service. I've been using several of them in a RAID 5 setup for years now with great results. But I don't feel as good about the newer drives as I did with the many dozens of lower capacity green drives I used in the past. I do still use several dozen of the old WD 1.5TB and 2TB green drives in two of my unRAID setups. But those setups are not on 24/7. Sometimes I will go a week without turning on some of my unRAID setups.
     
  18. trip1eX

    trip1eX imo, afaik, feels like to me, *exceptions, ~aprox

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    10-12 hrs of content is only 1 movie per day per tuner.

    For a household of 4 that is only 2.5-3 hrs of content per person per day.

    And what's every been the point of filling up a 1 TB drive let alone a 3 TB drive? It's never been because someone is going to watch all that content. It's about collecting stuff just to say you have it and just in case you want to watch xyz.

    But anyway imagine the suggestions this thing would store? Would it record 24/7 for a month if you kept suggestions on?
     
  19. Dan203

    Dan203 Super Moderator Staff Member TCF Club

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    I don't use suggestions. I record more stuff I want to watch then I can keep up with. And when I'm bored and don't want to watch anything I have recorded I watch Netflix, or Vudu, or HBOGo. I have zero use for Suggestions.
     
  20. nooneuknow

    nooneuknow Well-Known Dismembered Member (Lurk Mode On)

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    No, my budget doesn't allow me the luxury. But, as you say, the charts I've seen (datasheets and reviews) show more platters increasing the rate, in increments. Some drives are increasing the platter areal density, and adding platters, to hit 6TB.

    The difference between a 3TB and 4TB WD Green/Red drive is ~3MB/s. Beyond that, you have the same data I do. I have no itch to buy 6TB drives, and place so much trust in less, larger, drives. I waited for 4TB to be everywhere, before I went to 3TB. History seems to show that some drive capacities were inherently problematic, especially for the first to buy them. I like testing, but not enough to make a move past 4TB at this time.

    Since the TiVo Mega speaks of 10x3TB drives, I was posting assuming it used 3TB drives. I probably should have made a mention that I was posting based on using 3TB drives.
     

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