[mythtv-users] backend storage performance needs?

[Stephen Worthington]

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.