[mythtv-users] Planning Tuners and Disk I/O for a MythTVBackend System

[Jim Beckett]

Raymond Wagner wrote:
> The innermost track of a 1.5TB 5400RPM drive is going to push 50MB/s 
> sequentially.  With some safe estimates of 30 seeks/sec, and a 15ms 
> seek latency, almost half the time would be spent seeking, dropping 
> throughput to 25MB/s (ignoring onboard cache and NCQ).  Those twelve 
> recordings each at a respectable 16mbps will use 24MB/s.  That puts it 
> in the realm of possible, but probably not without problems.  Of 
> course if you own 12 tuners, you should have no problem getting 
> multiple hard drives to record to.
>
>
> ... Striping causes the same exact problem.  You still have all the 
> disks seeking in unison, you just end up with significantly higher 
> throughput.  If you've got proper hardware RAID with write-back cache, 
> it may be able to re-order the writes to be more sequential.  The 
> whole purpose of Storage Groups added in 0.21 was to allow you to add 
> multiple independent drives to MythTV, and allow it to properly load 
> balance between them.
>
I am appreciative of all the responses that I have received on this 
topic. (Although, I'm extremely embarrassed that the initial set of 
'facts' I was using were so horribly incorrect.)

This discussion has illustrated some things about my current storage 
system that are probably just flat wrong, and that I should fix, before 
I even think about the requirements for a new system down the road.

First the background, and then a load of questions at the end...


One mistake is that all of my partitions are ext3, and I should probably 
change the recording partitions to xfs, or another file system that 
handles large files better.

(I haven't noticed any issues with the current ext3 file systems I have 
in place, but that could be due to inattention and dumb luck, as much as 
anything. I think I should fix this for good measure, if nothing else.)


Another mistake I think I have made was using my USB2 500GB drive for an 
additional recording storage area.

I think that this recordings storage area should probably be moved to 
one of the internal SATA drives, and any music, pictures, and video 
storage groups, should probably move to the USB disk.

My thinking is that the primary use on the USB disk should be read 
mostly, and would only see occasional write activity on it.


The last mistake I see is that the way I have these disks partitioned is 
a mess (IMO):

Filesystem            Size  Used Avail Use% Mounted on

/dev/sda1              55G  2.7G   49G   6% /
/dev/sda3             138G  188M  131G   1% /livetv
/dev/sda4             267G  154G  100G  61% /recordings

/dev/sdb1             3.8G   97M  3.5G   3% /pictures
/dev/sdb2             376G  169G  189G  48% /videos
/dev/sdb5             9.4G  2.9G  6.1G  33% /music
/dev/sdb6              68G  822M   64G   2% /datashare

/dev/sdc1             459G  325G  111G  75% /recordings2


So in order to clean up my storage issues, I think I would probably want 
to have it set up like this, and used balanced IO in MythTV for recordings:

/dev/sda (first internal disk) - 500.1GB SATA  ~73 MB/s disk reads

    sda1, 60GB, mount point /, Ext3
    sda2, 400GB, mount point /recordings, XFS

/dev/sdb (second internal disk) - 500.1GB SATA  ~73 MB/s disk reads

    sdb1, 4GB, swap space
    sdb2, 56GB, mount point /livetv, XFS
    sdb3, 400GB, mount point /recordings2, XFS

/dev/sdc (USB 2.0 - 500.1GB External drive) ~31 MB/s disk reads

    sdc1, 400GB, mount point /media/disk, FAT32 ***
    sdc2, 60GB, mount point /datashare, FAT32


(*** will hold videos, music, and pictures, and I assume I'll need to 
fix these locations in Myth, and Samba)


Does this look like a better use of disk space for a dedicated FE/BE 
MythTV system?

Is there any performance increase/decrease that would mean I should do 
this differently?

Will the new sda1 partition be large enough, or is 60GB too big?

I see XFS, and EXT4, talked about a lot for large file types. Which of 
these, or possibly another, would be the best choice for the recordings 
directory?

I'm thinking FAT32 on the USB drive for portability, if I ever want to 
mount it temporarily on another system. Shouldn't be a problem for 
MythTV, should it?

Lastly, I have 2GB of RAM installed in this system, should I even bother 
with the swap partition?

Any advice, guidance, or constructive criticism is welcome!


Thanks,

-JIm