Re: [Discuss-gnuradio] Gig-E alternatives

[Eric Blossom]

On Thu, Jun 23, 2005 at 07:38:01AM -0400, Lamar Owen wrote:
> On Wednesday 22 June 2005 20:36, David P. Reed wrote:
> As to throughput,  you are still limited in IO bandwidth at the PC level, 
> particularly in the HD interface.  While the interface itself might be 
> substantially faster than the necessary throughput (ATA133 supposedly can 
> transfer 133MB/s, SATA 150 can do 150MB/s) in reality the HD platter transfer 
> rate is typically much lower: my ATA100 drive in my laptop, for instance:
> address@hidden ~]# hdparm -t /dev/hda
> 
> /dev/hda:
>  Timing buffered disk reads:   70 MB in  3.05 seconds =  22.95 MB/sec
> address@hidden ~]# hdparm -i /dev/hda
>  
> This means that there is no way I can store data at the current USB rates.  
> My 
> faster server could:
> address@hidden ~]# hdparm -t /dev/hda
> 
> /dev/hda:
>  Timing buffered disk reads:  172 MB in  3.02 seconds =  57.04 MB/sec
> address@hidden ~]#
> 
> But even then it would be hard pressed to keep up for long.  The server has a 
> fast ATA133 Maxtor 250GB drive as /dev/hda, and is one of the faster ones 
> around.  EVEN THEN I could not keep up with full-bore GigE.

Laptop drives suck for throughput. 

Most of the current big, cheap, 7200 RPM desktop drives will sustain
40MB/sec over SATA.  If you stripe two drives RAID 0 using LVM or
software raid, you should be able to sustain 80MB/sec.  You'll never
sustain this with ext3 because of journal posting, but ext2 or xfs
should work fine.

While doing initial testing it's useful to have a raw partition to play
with, to remove the filesystem from the equation.  Once you know what
the drives and will really do, find a filesystem that will work.

> Next, to fully take advantage of GigE, you have to have multiple PCI buses in 
> your system, if you have 32 bit PCI especially.  32 bit PCI at 33MHz rate is 
> capable of 133MB/s max;  GigE at full rate is very close to this.  Many if 
> not most GigE cards have a 64 bit PCI-X interface;  66MHz or better 133MHz 
> PCI-X at 64 bits is capable of up to ten times the 32 bit PCI rate, and is 
> typically only available in servers.

Lots of the lastest motherboards have SATA built in, usually pretty
high in the bus hiearchy.  Many of them have PCI Express, and there
are many SATA controllers for that.

Eric