[mythtv-users] Anyone using SSD for long term storage?

[Joseph Fry]

>
>
>  Note, I tried recording 5 HD streams to a single spinning disk, and while
>> it seemed to work, I could hear the drive thrashing.  Spreading the
>> recordings over 3 smaller drives made a huge difference, especially because
>> I have peaked at 8 HD recordings at once.
>>
> Honestly I don't see point to make things more complicated than required.
> Adding additional drives because "I could hear the drive thrashing" seems
> to be little surprising for me.
>

I didn't actually "add" drives.  My system consisted of 3 500 GB drives and
a single HDHR recording from QAM... I purchased a HDHR Prime and a 3TB
drive, with the intent of moving everything over to a single drive.  I
tested 5 recordings from my HDHR (was waiting on my cablecard) to the
single drive and simply didn't feel comfortable with it due to the obvious
thrashing on the drive... and I knew that once I got the Prime up and
running it would just get worse.  So now my system has 4 drives in it...
the 3 500's are used for recordings only... everything else is on the 3TB
drive.


> Using latter jobqueue to reshuffle recordings....oh my Dear - I understand
> setup of this was nice brain training during those winter evenings :-)
> MythTV is pretty effective with multiple sustained writes to storage. I
> done many tests with 16xHD in single HDD (WD Green 3TB) - no any problem.
> Sure - HDD LED was constant ON. But HDD role is write/read - so as long as
> IO wait is reasonable - I don't see issue.


I did the math, and assuming an HD recording is about 4GB / hour... it's
only 17MB/s to write 16 recordings to a single disk... certainly reasonable
that a single disk can do it.  But with two comflagging jobs running and
simultaneous playback of a couple of recordings, the heads are gonna be
going nuts.  A single SSD doesn't have that issue.

Perhaps my 3 recording drives are overkill... but I am more comfortable
with this arrangement.


>
>
>> For the database, you can see a substantial improvement unless your mysql
>> configuration is heavily optimized to eliminate disk reads.
>>
> This is interesting.
> Pls look here http://www.gossamer-threads.com/lists/mythtv/users/519664#
> 519664
>
> Exec summary from above link:
>
> 1.In case of my system build-in-kernel disk buffering is so effective
> that even significant speedup of next level storage hierarchy (HDD->SSD)
> is totally masked by kernel buffering.
>
> 2.Big enough MySQL caches quite effectively masks speedups in mass-storage.
>
> 3.Recordings rescheduling speed is directly correlated to CPU (going
> from Celeron 3,46G to AMD 235E changed reschedule time from 60s to 18s).
>
> 4.For rescheduling speed-up, next, after CPU upgrade & mysql caches, is
> moving /tmp to tempfs (noticeable speedup 18s->6s).
>
> 5.Current MythTV system based on quite standard HDD like WD Green is
> working really nice even with ATOM CPU. (i.e.86 progs EPG list loads in
> aprox 0.2-0.4sec; showing 400+ recordings list is instant (in sense that
> there is no time to show loading progress dialog).
>
> Summarizing:
> Speaking in economy: HDD->SSD marginal utility increase is in my case
> was ABSOLUTELY not worth of SSD investment.
>
> Before SSD spending decision I'm strongly recommending:
>
> -buy more RAM for having huge as possible kernel disk buffers
> -move /tmp to tempfs
> -allocate 32MB or more for MySQL query cache (and other parameters
> accordingly to http://mysqltuner.pl/)
>
> With above it is highly probable that You will get MythTV system where
> HDD->SSD upgrade will absolutely not worth the money (and allows You
> spent those money elsewhere - with much higher utility).
>
> YMMV


On that same thread, others clearly stated that they did see significant
performance increases going to SSD.  I agree that you may be able to negate
most of those increases by ensuring you have enough RAM for a large disk
buffer, moving /tmp, and configuring mysql using mysqltuner.pl and a large
query cache.

Regardless... an SSD is a step up from spinning media if cost isn't your
primary deciding factor.  I still think use as a recording drive is
reasonable due to the reduction of access time and latency, and elimination
of any impact from fragmentation; if you routinely run your recording
drives nearly full, you will get fragmentation regardless of filesystem
used, the fact that recordings are so large and your doing multiple
recordings at a time only increases the chance of fragmentation.

Is it a worthwhile upgrade when cost is a deciding factor... probably
not... which is why I don't have any SSD's in my system.
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