[mythtv-users] backend storage performance needs?

[Stephen Worthington]

On Tue, 19 May 2020 11:03:36 -0400, you wrote:


>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

An SSD's lifetime is normally measured in Tbytes written.  If you want
to record to an SSD, you need to do the calculation of how much data
you will be writing to the SSD to do the recordings, versus its rated
lifetime.  I did that for an NVMe M.2 SSD when I was getting ready to
install one, but the numbers then were not good.  If I were to do my
recording to my SSD, it would expire in less than a year.  As it is,
with the system and database on the SSD, it is going to be 20-25 years
before it will die, which means it will likely be replaced long before
then.  That was nearly four years ago - your needs are smaller and
SSDs are better (and much bigger, which gives a larger Tbytes written
value).  So that calculation may say that it is a good idea now.
Getting rid of the head movement problem is great, but SSDs have their
own limitations.

If you are going for a new motherboard, then get an NVMe SSD.  Almost
all motherboards now have at least one, often two M.2 NVMe slots.  The
maximum speed of a SATA interface is way below what an SSD's hardware
can do.  Good NVMe SSDs are 10 times faster than a SATA one, and do
not cost that much more.  And they seem to be cheaper every month.
With a SATA SSD, you would also have to calculate whether multiple
recordings plus database activity plus the usual system activity would
be too much for a single SATA 6 Gbit/s interface.  There are no such
worries with NVMe.