[mythtv-users] What is best filesystem for recordings?

[Andre Newman]

On 30 Jun 2015, at 20:33, Warpme <warpme at o2.pl> wrote:

> I was thinking over week: should I go with 8T from Seagate.
> I took risk :-p

Good luck ;-)

A friend of a friend works for Seagate and even after staff discount he doesn’t recommend their own drives! I even had grief with a Enterprise Seagate SAS drive recently, I was a little worried when the client handed me a Seagate drive but oh well, he’s the client, he said he was sure.

> I copied already over 2.9T movies to this drive. It performs surprisingly well with write speed: top is 120MB/s, avg 90-95MB/s.
> Unfortunately I didn't check speed sustainability - but I'm expecting not good results here.

What happens when you do rewrite? Does the drive cache the rewritten tracks until it gets to the guard band? You say you have no UPS/APS/USV, then what happens if the power goes in the middle of a rewrite? What is the point to worry about the file system being power fragile if the drive is extremely power fragile underneath? I know you don’t use this for recordings but it’s in the same machine.

> 
> Mythvideo storage is perfect candidate I think.

I agree, I expect I will get one some time, maybe soon.

> HW was Intel DH67BL + i3220T.
>> This is a lot of simultaneous HD for only two drives! I usually think 4 per drive is absolute max, pref  3.
> 4? Why such benchmark?

Based on my tests with a margin, my tests were done some years ago when I had 4x 1TB Samsung drives. I noticed an improvement with a 3TB WD AVGP drive compared to a 4TB HGST desktop drive, 1TB HGST and 1.5TB Samsung. Since then I only buy 3TB or 4TB AV Drives for recording.

I have also noticed a big improvement in AHCI code in the Linux kernel, disk IO schedulers have been re-designed so I expect a more modern system is much more capable. I was making tests in Kernel 2.6 days and now we have 3.x and 4.x 

19 HD streams across 4 drives, 4+ per drive with a margin. I don’t have enough HD sources to re-do my tests (I cancelled my pay TV when it stopped working with MythTV) and multiple copies of the same channels has less randomness in the disk IO.

> 10 rec. with 6-7Mbps avg. each gives 60-70Mbps. Today average HDD has 600Mbps controller-to-platters. SATA has usually 3Gbps (sure, SATA1 has 1.5Gbps).

Yes but almost irrelevant for recording multiple streams, seek strategy and seek time is much more important and this is where the AV drive claims to be different. WD spec for many more recording streams but with CCTV bandwidth, these are also the drives that the PVR manufacturers use, I think there is a reason, ok slow low power spin up and low heat are also good reasons but they are good reasons for MythTV too.

Really I should compare otherwise identical drives in a proper test, I did more recently try a 4TB WD Green that a friend lent me and it didn’t do as well as my existing 3TB AVGP but I didn’t persist with full testing because what I have works for what I need. That test was also not with MythTV so the MythTV file syncing was not a factor, I think this has been relaxed a little in recent versions too.

> 
> When I was looking on whole chain (DVB-myth-OS-HDD) bottleneck was random write to HDD.

Exactly.

> With NCQ I had IOwait about 45-55% with 16HD streams, VERY rough estimation means something like 30-32HD streams is limit.
> Having better random write optimization that 32 stages NCQ will help to get more.
>> I have a UPS but I have also pulled the wrong drive from the hot swap array! The XFS tools are different so you must learn them.
> This is another argument for EXT4 over XFS - albeit subjective one :-)

I come from using XFS on SGI so for me XFS is more familiar.


>>> 4.stability/maturity (i.e. recovery from bad/deep FS corruption);
>> As above, learn the tools, ask the mailing lists but it’s worked well for me for many years.
> Here i think EXT4 is better choice. It is default in many distros so probably most tested and verified....
>> 
>>> 5.any other I forgot..
>> A friend runs a simultaneous recorder system (he uses MythTV for this) where he records all the main UK DVBT channels HD & SD from seven tuners on to six harddrives, he says ext4 is useless for this but xfs works fine.
> This is VERY interesting info. Is Your friend investigate why EXt4 goes so badly?

Not thoroughly, he tried some mount options.

> Wasn't his EXT4 issue related to use-case where FS is asked to write and delete in real-time on the same time?
> it will be good to understand this!
> Why he goes with XFS and not with JFS?

I suggested XFS, it worked, he went back to other hobbies.

Andre