[mythtv-users] Can't get storage priorities correctly.

[Ram Ramesh]

On 11/17/23 21:23, Stephen Worthington wrote:
> On Fri, 17 Nov 2023 17:35:49 -0600, you wrote:
>
>> On 11/17/23 10:59, Ram Ramesh wrote:
>>> I have two storage directories of vastly different sizes (1 TB SSD and
>>> 10+TB spinning RAID1).  I had them both in default group and balancing
>>> algorithm as "combined" with default setting. I thought that myth is
>>> smart and will figure out and favor SSD over spinning disk when
>>> possible. That did not work. So, I changed the algorithm to "Balanced
>>> free percent" and was hoping it will keep about the same % of free
>>> space in both disks. I noticed today that SSD has about 13% (96G)  use
>>> (87% (660G) free) and spinning disks have about 94% (12T) use (6%
>>> (770G) free).  This means next recording should go to SSD. However, it
>>> is now writing to my spinning disks. What did I do wrong?
>>>
>>> Distribution: Debian bookworm (Linux 6.1.10)
>>> Myth: 31+fixes from debian-multimedia
>>> (31.0+fixes20201214.gite9b795a1e4-dmo0~bpo10+1 as per apt)
>>>
>>> I only changed the storage priorities yesterday and the new recording
>>> is the first one after that. Do I need to wait or reboot after
>>> changing storage balancing algorithm before it takes effect?
>>>
>>> Regards
>>> Ramesh
>>>
>> Found the problem. I had not started mythtv-setup with sufficient
>> privilege to  kill the running backend. So, even though it reported that
>> it stopped backend, it did not. Thus my update did not make it to
>> backend at all. Once I restarted backend manually, it seem to work as
>> expected.
>>
>> I wish backend-setup caught this error and failed instead of making me
>> believe that everything is fine. Anyway, I am running an older version,
>> and may be it is fixed already.
>>
>> Regards
>> Ramesh
> Have you done the calculations for the extra usage that MythTV will be
> doing for the SSD?  When I looked at using an M.2 SSD for recording,
> the calculations showed that it would be worn out pretty rapidly, due
> to the recording files being large.  The lifetime of an SSD is
> normally in TBytes written - writing multiple multi-Gbyte files per
> day is a good way to wear one out.
>
> Do you have a particular reason for wanting the recordings done on the
> SSD?  Or is it just that you need to record more things at the same
> time than can be done with one hard drive?
>
> You also need to look at the SSD's characteristics.  A lot of SSDs
> have cache, and when the writes exceed the cache size, they slow down
> a lot, until the cache can be written to the actual flash storage in
> the SSD.  With the large file sizes for recordings, the cache size can
> easily be filled, particularly if you are recording more than one
> programme at once to the SSD.
>
> So if you are just needing to make more recordings at the same time, I
> would recommend investing in another hard drive, rather than using
> your SSD.  Or do the lifetime calculations and make sure to get a new
> SSD as soon as the old one is getting low on lifetime - that may be
> every year or two.
>
> And make sure you install smartmontools and use the smartctl command
> to see what the lifetime writes are and what the SSD is reporting as
> its remaining lifetime or % used or whichever way it reports that.  I
> have smartctl set up to monitor all my drives (SSD and spinning rust)
> and report any problems via email and popup messages.  This has saved
> me many times when a drive has started to die.
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Thanks for your concern. I thought a  lot before choosing this path.

Yes, I did my calculations. My SSDs have 600TBW and at 30GB per day, I 
am looking at 54 years of life (600*1000/30/365 ~ 54). I also have 
smartd doing its thing. It will report when SSDs start using their spare 
blocks.  At about $60/TB (for 54 years of claimed life), I feel it is a 
fair game.

So far, in my mythtv usage (10+ years), I have had 10+ spinning disks 
die. I have still my very first SSD (256G SATA) and several others  as I 
have no use for them with newer/larger  SSDs replacing the old ones.  
So, I am looking to try them out to see if TBW values really happen.

Suppose I am wrong and my SSDs show wear, it is a single mythtv-setup 
change that will get back to trashing spinning disks.

When I record and watch at the same time, the amount of seek is very 
noticeable (especially with RAIDs) and I prefer the random access 
capabilities of SSDs.

Regards
Ramesh
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