[mythtv-users] read-only disk in a storage group?

[Stephen Worthington]

On Mon, 8 Jun 2015 10:43:03 -0700, you wrote:

>On Mon, Jun 8, 2015 at 10:37 AM, Brian J. Murrell <brian at interlinx.bc.ca> wrote:
>> If I had a storage group with a disk that I wanted to be able to read
>> from but not write to, can I tell MythTV to do that?
>
>Just create a new Storage Group, add that disk to it, then never
>configure recordings to use that Storage Group. Recordings will use
>the Default group by default. Call your new group something like "Read
>only" to remind yourself not to choose it for recordings. Anything on
>that disk will still be available for playback.
>
>Tom

Yes, that works.  I have just got my first 8 Tbyte drive, a Seagate
ST8000AS0002 "archive" drive, which uses SMT technology.  SMT uses the
fact that the read heads on a drive can be significantly smaller than
the write heads, so it overlaps the writing of tracks onto the side of
previous tracks.  Reading is OK due to the smaller read head, but
writing a track where there is already data then requires rewriting of
all the data from there onwards until the drive reaches a guard band
where it does not overlap the tracks.  As a result, if it has to do
the worst case rewriting, the write speed can drop to zero for some
time.  So SMT drives are quite unsuitable for recording to as the
write speed can vary from pretty fast (120-140 Mbyte/s) down to 0
without notice.  But SMT makes for a much cheaper drive for the
storage size.

So am using this drive for archiving recordings - moving them off my
real recording drives to the archive drive.  I have a storage group
called "archives" defined, where I put read-only drives, and all the
recordings on it show up just fine in mythfrontend.  The SMT drive on
its default settings shuts down to conserve power after a relatively
small idle time, but as soon as it has had its recordings directory
read, Mythtv or the system seems to cache that in RAM, so going to the
Watch Recordings screen does not normally wake up the drive and
happens just as fast as usual.  That does mean that playing one of the
recordings on that drive will take a while to start though, as the
drive startup time is significant.

I am pretty pleased about how it all works.  I have still to write and
update for my "balance_storage" script to automatically fill up any
partitions on my "archives" storage group, but I will get to that
soon.