[mythtv-users] Hard disk speed needs for MythTV system

[Dave Stewart]

As with a lot of things in computers, "it all depends...."

With modern interfaces, the usual limiting factor in data transfer is
how fast bits are going by the read/write head.  That varies with bit
density on the platters, number of platters, disk size, etc. etc.  Of
course, you only approach these maximum speeds when you're reading long
runs on a largely unfragmented disk, but this is typical for video.  

A fragmented file can invoke several seeks during a file, drastically
reducing overall bandwidth.  We did some experiments with 2 and 4 gig
SCSI disks several years ago, and files with more than 2-3 fragments
were significantly compromised.  For short (system) files, the limiting
factor is the finding the right block through the file system, not the
read bandwidth.  But I digress...  (Is anyone looking at defragmentation
in Linux and/or video4linux?)

But some generalities: on identical platters, 5400 rpm drives should
deliver data 54/72ths as fast, about 75%.  Also, since more bits can be
put on an outer cylinder than an inner cylinder due to the longer
circumference, the data rate is higher at the beginning of the disk than
at the end (inner tracks). I Googled a few references: A Western Digital
40GB 5400RPM drive was tested at ~30MByte/sec (outer tracks) and ~22
MB/sec (inner).  A WD 120GB 7200RPM drive was closer to 50 MB/s outer
and 35 MB/s inner.  (I don't know what the platter densities were.)

However, the implications are pretty clear.  If an older 5400 RPM drive
will deliver a 22 MB/second worst case stream, it *should* be able to
record two or three sub-1 MByte/second video capture streams, provided
the OS isn't doing something really dumb.  You could imagine a
degenerate case where the file system tries to interleave writing 1
block of each of 2 video files on opposite ends of the disk,
guaranteeing a worst case seek per block written. Personally, I know
nothing about the internals of the Linux file system, but I've been told
that it optimizes these situations correctly.

One other factoid: it's been years since disks were interleaved.  The
point of interleave was that in the olden days, the disk controllers
were not fast enough to read, decode and error check sequential sectors
on the disk.  Instead, a sector or more would pass under the head,
unread, while the controller worked.  Therefore the "next" sector was
not adjacent, but interleaved with one or more sectors that were part of
a different sequence. As controllers got faster, interleave eventually
achieved 1:1, i.e. sequential sectors.

Dave

On Fri, 2003-09-19 at 22:49, Itai Tavor wrote:

On Saturday, September 20, 2003, at 07:49 AM, Jarod C. Wilson wrote:
>
>Generally, there's not _that_ much of a difference between the two 
>when
>accessing large files, since the only thing you really get out of 7200
>IMHO is a different interleave which allows for faster random seeks
>(and
>anyone else is welcome to correct me if I have this wrong).
>
I think you're right about that. Both 5400 and 7200 are capable of
feeding video more than fast enough. I'm not sure at what point the
5400 would start to bog down, if ever (i.e., I'm not sure how it would
hold up if you had, say, a pcHDTV card, along with another tuner).
>
Anyone? Aaron? I use dual 7200s, RAID striped, myself. And my system is
still pretty quiet...
>
--Jarod

Thanks for the replies, Jarod and Aaron. Think I'll give a 5400 drive a 
go.