[mythtvnz] File System for RAID-5

[Stephen Worthington]

On Thu, 18 Jun 2009 13:30:55 +1200, you wrote:

>On Thu, Jun 18, 2009 at 12:57 PM, James Booth <james at booths.net.nz> wrote:
>
>>  Thanks Steve. RAID1 with LVM will give me what I need, I was just hoping
>> for RAID 1+0 so I could also get the performance gain, which may be handy if
>> I end up with three frontends all watching and recording HD at the same
>> time. If I had three RAID1 arrays as per Tim’s email, and then put RAID0
>> over the top, could I subsequently add in another RAID1 pair as per Tim’s
>> directions, and then grow the RAID0 across the extra RAID1 pair without
>> having to backup? I guess in short I’m asking can you currently add
>> disks/partitions to a RAID0 automatically without loss of data? I find many
>> conflicting answers on the net.
>>
>I wouldn't worry too much about the performance - hard drives are at least
>an order of magnitude faster than HD streams. I believe TV3 runs around
>11Mbps, a modern drive will easily handle in excess of 400Mbps. A two disk
>RAID1 should be capable of nearly twice the single disk read speed - when
>you've got multiple reads going (as its the case with streaming three files
>to different frontends) RAID1 should be approximately the same speed as
>RAID0 for the same number of drives.

You are missing the point a bit when you refer to disk speed only in
Mbit/s.  That is a useful number only when the disk is being used for
just one data stream.  If there are four tuners writing to the disk,
another recorded program being played, and the four new recordings are
being commercial scanned, then there are 9 data streams being used and
the disk heads are going to have to be moving between them all.  The
Linux caching algorithms are good, and will reduce the head movement a
lot, but there will still have to be head movement.  And while the
read and write speeds for modern 7200 rpm SATA drives are quite
staggeringly big, the stepping rates have not changed in ages and are
still a *lot* slower than even quite old 10k or 15k SCSI drives.

That said, my two Seagate 7200.10 500 Gibyte data drives combined as
one LVM/JFS partition cope well with the above scenario, but only at
MPEG data rates from my PVR-500 tuners.  I have yet to move my DVB-T
tuners onto the Myth box to find out if they will work as well when
there are two 1080i and two 720p programs being recorded at the same
time.

>As far as I know you can't extend RAID0 arrays with mdadm either (at least
>the man page says 1/4/5/6 only). My advice is that if you're really
>expecting to extend the array any time soon just buy the drive now.
>
>By the way, that stuff you keep referring to from Tim's email was also
>written by me :-)
>
>Cheers,
>Steve