[mythtv-users] Filesystem of choice for recording drive?

[Warpme]

On 21/01/15 20:51, John Drescher wrote:
> I consider that a bad choice. This drive is designed to limit the
> error recovery to a very short period (even shorter than a NAS drive)
> for usage in a video valence appliance where a few glitches in the
> data stream is okay but its bad to stop recording for more than a few
> ms.
This is interesting opinion.
I personally was thinking that WD Purple was designed for real-time 
non-data-integrity-critical apps where real-time has priority over data 
integrity. Such type is perfect for myth recording drive.

Let's consider scenario where HDD is concurrently writing recording and 
reading for playback.
Making real-time writing is doable (as drive can on-fly remap bad sector).
RT reading is much more difficult (as there is no concept of spare 
sectors for reading).

So lets imagine that over time one sector becomes bad and AV app wants 
to read this sector.
What will happen:

A: traditional drive (i.e. WD Green)
1.app reads sector
2.CRC errors discovered
3.HDD controller rereads bad sector multiple times with hope that drive 
CRC algorithm redundancy data will recover error.

B:AV streaming optimized drive (i.e. WD Purple)
1.app reads sector
2.CRC arrors discovered
3.HDD rereads sector only few times with hope that drive CRC redundancy 
data will recover error. When fails - read operation ends with success 
but HDD returns bad data!!!

Now:
A.3 can be 10..60sec or even more.
B.3: can be let say 0.5sec (time in which HDD cache can buffer incomming 
write data when write to platters is blocked by read recovery)

As 3 is write blocking, we can say:
WD Green will damage recording for hope of getting glitch-free playback
WD Purple will try keep recording with price of glitches in playback

What is better for myth recording drive?

I believe, for AV apps (where lack of data integrity is far less 
negative than breaking real-time regime) WD Purple is much better solution.

For OS/DB/apps execs WD Purple is much _worse_.

IMHO WD correctly segmenting by trade-off between real-time vs. data 
integrity here.

I think advent of class of AV streaming HDDs are answer for this 
trade-off and I think it is good that HDD vendors are segmenting in such 
dimensions instead of other (usually much more artificial)

br



>
>
> John
> _______________________________________________
> mythtv-users mailing list
> mythtv-users at mythtv.org
> http://lists.mythtv.org/mailman/listinfo/mythtv-users
> http://wiki.mythtv.org/Mailing_List_etiquette
> MythTV Forums: https://forum.mythtv.org
>