[mythtvnz] SSD Drives for video

[Stephen Worthington]

On Sun, 8 Jun 2014 17:16:20 +1200, you wrote:

>On Sun, Jun 8, 2014 at 4:34 PM, Worik Stanton <worik.stanton at gmail.com> wrote:
>> On the subject of using an SSD for recording onto...
>>
>> On 05/06/14 19:28, Pieter De Wit wrote:
>>> Personally, I would avoid this as the ssd will wear out
>>
>> I expect that Pieter is thinking of memory wear
>> (https://en.wikipedia.org/wiki/Flash_memory#Memory_wear).
>>
>> I am using my 30G SSD drive while my net-storage device is in the shop
>> getting fixed.  I do not believe I am in danger of encountering problems
>> with my SSD in this period.  But have been thinking that I will not be
>> buying mechanical discs again.   Pieter's comments are food for thought
>> and it is worth examining the issues.
>>
>> http://www.snia.org/sites/default/files/SSSI_NAND_Reliability_White_Paper_0.pdf
>> says:
>>
>> "Another peculiarity of NAND Flash as a storage medium is its increasing
>> susceptibility to bit errors after having gone through a certain number
>> of program-erase (P/E) cycles.The number of cycles varies, depending on
>> density (storage capacity per physical area), vendor, and NAND Flash
>> type. Generally speaking, SLC is usually rated for ~100,000 P/E cycles,
>> whereas MLC Flash is usually rated for ~5,000-10,000 P/E cycles. NAND
>> Flash also becomes less reliable over time when unpowered."
>>
>> SLC is the older variety and MLC the newer variety, AFAICT.  Odd that
>> newer varieties should be less durable.
>>
>> But for our purposes, recording video, surely even 5 to 10 K cycles
>> would be OK?
>>
>> Cost is clearly an issue, with TB hard discs being so cheap c/f SSD, but
>>  what are people's thoughts on using SSDs in Myth?  Is it worth the
>> extra cost?  Is it worse than spinning discs (due to the limited P/E
>> cycle)?
>>
>> What are people's experiences?
>
>I have no experience of them, but the perceived wisdom of what I have
>gleaned from the mythtv-users list is that they are worthwhile for
>doing initial recording IF you have high volume multiple stream
>recording requirements, eg you want to record 10 streams on a fairly
>continuous basis (and also perhaps if you are doing a lot of live tv,
>where you not only have to write but also read).
>
>But most people that I have seen with that type of setup transfer the
>recording to a spinning platter pretty soon after recording. For
>example you can set your storage groups up so that all recordings
>initially go to disks 1 and 2 (which will be ssd) but clients will
>look on disks 1,2,3,4 and 5 for recordings, so the move from disk 1
>(SSD) to disk 3 (normal disk) will be transparent to the end user.
>(Not sure what happens if you move something while someone is actually
>watching it!) Disks 3-5 may of course be in the same machine, or on a
>NAS.

If you are recording 10 streams at once, then you can just have 5 hard
drives and that will solve that problem.  If you are recording 10
streams at once, then you will likely need all the extra hard drives
to store all those recordings anyway, so it does not make so much
sense to store to SSD first and then copy to a drive you could have
written to directly.

I have 6 recording drives now, and I am recording 4 programmes
currently.  I am fairly sure I have had times when I am recording 6 or
even 7 programmes at once, but even if they were all overlapped by new
recordings directly afterwards (14 streams at once), with 6 drives I
would not expect any problems.  I also have 8 Gibytes of RAM which
helps too, as mythcommflag runs in real-time from the RAM buffers,
rather than doing disk accesses.

Where SSDs are good for MythTV is for the database.  When the
scheduler runs, it uses a huge number of database accesses very
quickly, and an SSD can speed that up enormously.  My scheduler times
are getting annoyingly long, so I am contemplating an SSD, but can not
afford one quite yet.

The latest and fastest SSDs use a PCIe bus connection, as they are now
too fast for the old SATA specifications.  The latest SATA
specification update includes connecting drives that way.