Re: Large Sequential Reads Being Broken Up. Why?

[James Smart <James.Smart@Emulex.Com>]

Here's one thing that will help you....
   in the lpfc driver (in rev 8.0.13 - the file is lpfc_fcp.c), in the
   scsi_driver_template structure - add the field:
       .max_sectors = 0xFFFF,

As of 2.6.10, the kernel started paying attention to this field, which the
emulex driver, as of that time, didn't set. The result was the kernel
dropped back to a default max_sectors of 1024 - which results in a 512k max.
The lpfc driver was updated in rev 8.0.29 with this change.

Caveat is : Even with this change, you must be using O_DIRECT to get high
bandwidth. Otherwise, the upper layers will segment the requests (if I
remember right, we had a hard time making a "normal" config exceed 256k).

-- james s

Martin W. Schlining III wrote:
> I am running a program on my Linux box which is asking for 2M IO (reads 
> and writes) with the file handle being opened with the O_DIRECT flag. 
> However, the IO being put out on the wire is no larger than 512K.  My 
> target device is the SCSI block device (/dev/sdb in this case). What is 
> preventing me from getting large IO through the SCSI block layer? How 
> can I fix it?
> 
> The sg device can achieve the 2M IO size, so I know its at least 
> possible. How can I improve the IO size for the SCSI block layer?
> 
> Details:
> 
> Dell 2850 server with dual Xeons, 1G RAM
> OS: Linux racerx 2.6.11.4-21.10-smp #1 SMP Tue Nov 29 14:32:49 UTC 2005 
> x86_64 x86_64 x86_64 GNU/Linux
> Emulex LP11000 Fibre Channel HBA using driver version 8.0.13 (changing 
> the driver hasn't helped, so far)
> I set the lookahead value pretty large to improve read performance 
> (hdparm -a)
> The scheduler for this device is anticipatory.
> 
> Any ideas?
> 
> Thanks,
> Martin Schlining
> 
> 
> 
> 
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