[mythtv-users] backend storage performance needs?

[blind Pete]

On Tue, 19 May 2020 11:03:36 -0400
James Abernathy <jfabernathy at gmail.com> wrote:

> On Tue, May 19, 2020 at 10:04 AM Stephen Worthington <
> stephen_agent at jsw.gen.nz> wrote:  
> 
> > On Tue, 19 May 2020 06:44:41 -0400, you wrote:
> >  
> > >Just an opinion request here.  When I built my current backend it
> > >was an old motherboard with lots of SATA II (3Gb/s) ports. I was
> > >worried about the need to have 2 recording drives defined so 2
> > >simultaneous recordings would go to different drives.
> > >
> > >I also wanted RAID mirrors so I used 4-2TB drives with mdadm RAID
> > >1. A lot to maintain as drives do fail and RAID volumes have to be
> > >replaced and rebuilt. Replacing and rebuilding has occurred on
> > >average of once a year.
> > >
> > >So the question is, if I bought a new motherboard with SATA III
> > >(6Gb.s) ports and new SATA III high performance hard drives would
> > >I really need to define 2 recording drives? i.e. use 2-4TB drives
> > >in RAID 1 mirror configuration.
> > >
> > >My tuner is a PCIe Hauppauge WinTV Quat HD. I have 4 tuners
> > >defined with each tuner having a maximum recordings of 2. But I've
> > >never recorded more than 4 channels at once with some back-to-back
> > >programs on the same channel.
> > >
> > >With the current speed of processors, serial connections, and hard
> > >drives do I still need 2 recording drives defined?
> > >
> > >Since SSDs and RAID is not recommended, I avoided that in the
> > >discussion.
> > >
> > >Jim A  
> >
> > There is no exact answer to just how many recordings at once you can
> > do with one hard drive.  There are quite a number of factors to
> > consider.  For example, if you are recording from all four tuners,
> > and you have back-to-back programmes scheduled for all four tuners,
> > you can suddenly be recording eight programmes at once for the
> > overlap period.  On top of that, you may also be playing back one
> > recording per frontend.  And as you get more locations on the disk
> > where there is active writing (or reading), that means the heads
> > will be spending little time on station before they have to move
> > again to the next active location.  It is not a linear problem -
> > going from four to five active locations uses far more disk
> > resource (available head movement) than going from one to two
> > active locations.
> >
> > Then you get the problem of the filesystem.  Each recording will get
> > to a point where it needs more space to be allocated.  So the heads
> > need to move from where the recording is to the system areas to find
> > and allocate that space.  That may be fine when it happens for one
> > recording, but if it happens for all four or all eight at once,
> > suddenly the heads are moving great distances back and forth and may
> > be too late getting back to where the recording is being written.
> > Filesystems vary as to how they handle that - some have the system
> > areas spread out across the disk to allow shorter head movement.
> > Some have all the system areas at one end of the disk.
> >
> > Then there is the organisation of the disk into tracks and
> > cylinders. A disk with more platters and fewer tracks per platter
> > has less distance to move the heads between things.
> >
> > Then you need to consider what may happen when the drive is getting
> > full.  If the filesystem is not good at keeping the free space
> > contiguous, then the heads will have to move much longer distances
> > between the files.
> >
> > Modern fast hard drives can have tremendous sequential write speeds
> > - those have been increasing all the time as the rotational bit
> > densities increase.  But the speeds of head movement have not
> > increased in proportion.  So they tend to be the limiting factor,
> > not the write speed.  And the drive manufacturers are much less
> > forthcoming about stepping rates and the like - they like to
> > advertise the super sequential write speeds.
> >
> > And then there is the cache - a large RAM cache and a good caching
> > algorithm (where the head movement is minimised) can make a big
> > difference when the heads have to move away from a recording to
> > update the system areas.  The updates for all the open files can be
> > delayed and then all done at once, rather than the heads moving
> > back and forth.
> >
> > Personally, I am quite conservative with the number of drives versus
> > number of simultaneous recordings.  Part of that is because I have
> > several older drives (3 x WD Green/Red, 6-7 years old), but also
> > because when I started out doing lots of recordings at once, I did
> > get occasions where I had all the recordings I was doing
> > simultaneously fail or be corrupted due to exceeding the ability of
> > my (then two) drives to handle the load.  So now I have seven
> > recording drives, and am upgrading them slowly to much more modern
> > and higher capacity.  I plan on no more than two recordings at once
> > per drive.  I do regularly (at least once a week) do 10-12
> > recordings at once, and with that setup I have never had a problem,
> > even when the load increases sharply (potentially doubles) during
> > back-to-back overlaps.  I only use normal drives, not RAID.  I can
> > not afford the extra costs of doing RAID.
> >
> > Except for their noise, my favourite drive is now the Seagate Exos
> > series enterprise helium drives.  I have an ST12000NM0007 (12
> > Tbytes) and an ST14000NM0018 (14 Tbytes) in my main MythTV box
> > now.  They are amazingly fast, and are good at handling transaction
> > processing where the data can be spread across the entire disk.
> > And they are much cheaper than anything else with the same
> > performance - they are actually cheaper here than the high grade
> > NAS rated drives.  As enterprise drives, I expect them to have a
> > long lifetime, but I have yet to find out just how long they will
> > last - only time will tell.  I have been very happy with all the WD
> > drives just keeping on going for 8+ years.  And Seagate did make
> > some horribly bad drives with very high failure rates for a while -
> > I had 5 of those drives all die early (< 2 years) and only one of
> > them that lasted, and that only for about 5 years.  Fortunately,
> > they were mostly so bad that they died while still under warranty,
> > and under New Zealand law I was able to refuse to accept a
> > replacement with the same type of drive as they clearly were not of
> > an acceptable standard of durability.  I got very good at
> > recovering the data onto the replacement drive, so I only lost
> > maybe 10-15 recordings from all of that, as the mode of death was
> > to get more and more difficult to read sectors, rather than the
> > entire drive just collapsing and all the data being gone.
> >
> > If you can afford them, even better than the Exos drives are the
> > Hitachi (now WD) helium drives.  Unfortunately, their price is
> > significantly higher for the same capacity - maybe 40% more.  But
> > the Hitachi engineers have always made very reliable drives.  The
> > two three Tbyte ones I retired when I upgraded to my Exos drives
> > had been in service 24/7 for eight and nine years respectively.
> > And one is back in service on my mother's MythTV box as she ran out
> > of space.
> >
> > Do not buy the cheap "desktop" drives - they are unsuitable for 24/7
> > operation and will die early if used like that.  You want at least
> > NAS rated drives.
> >  
> 
> Thanks for your thoughts.
> 
> I'm wondering if instead of RAID 1, I should just use 1-2TB SSD as a
> recording drive and then once a night do a cronjob that rsync's the
> recordings to an external drive for backup. I really only have about
> 800GB of actively use space for recordings because once my wife and I
> have seen them we delete them. Not building a library.
> 
> I was doing the rsync part in preparation to upgrading to mythtv V31
> in case I could not just move the current RAID.
> 
> Also I know that I can record 4 HD programs while watching a
> recording all at once because I did this as a test on my RPi4, HDHR
> Quatro, and USB3-to-SATAIII 1TB SSD.
> 
> The RAID needs are because I also use my backend as a NAS for other
> data, mostly backups.
> Then my drive list would be:
> 
>    - 1-2TB SSD as boot drive and Mythtv Recordings
>    - 2-2TB HDD as RAID 1 mirror for general NAS use for home
> computers.
>    - 1 external 2TB HDD drive to backup recordings daily.
> 
> That would take head movement out of the equation and at most I'd
> lose one day's recordings.
> 
> Jim  A

If you want to add another layer of complexity, set up an LVM and use
one of the SSDs as a cache for the RAID array.  My suspicion is that
even if you manage to thrash that SSD to death, it will take you ages to
notice. 

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bP