[6168] in linux-scsi channel archive
Re: how to get maximum datarate from SCSI
daemon@ATHENA.MIT.EDU (Mateu Batle i Sastre)
Wed Mar 24 18:56:37 1999
Date: Thu, 25 Mar 1999 00:36:49 +0100
From: Mateu Batle i Sastre <mbatle@iname.com>
To: Dietmar Stein <dstein2203@t-online.de>, linux-scsi@vger.rutgers.edu
Hi,
> You can mount your disks with sync-option in order to go around the
> buffer/cache. Have a try (e.g. mount -o sync /dev/sda1 /mnt) and compare the
> performance.
> I think you will go back to the async-mode very fast...
>
I think that's only to sync written buffers immediatly, but the transfers still
go through
the buffer cache.
The problem with the application we're developing is that we need high
data transfer, more than 20 MBytes/s or even more. This can be achieved
with an Ultra Wide SCSI adapter and several good SCSI hard disks.
This data is only transfered from disk to a buffer in a PCI device and
viceversa.
Actually, the CPU does not need to process this data. The buffering of *all* the
transfers is bad for this kind of applications, the CPU utilization reaches 100%
and it is still not enough to accomplish our goal. In this case, the CPU bounds
the transfer datarate. Additionally, in this type of applications use the buffer
cache has the problems of: very high CPU load, thrown of the
really useful cached buffers, and the same with CPU caches.
The same system with Windows NT runs perfectly because NT has a way
to do this I/O directly. It has a flag FILE_FLAG_NOBUFFERING that can
be specified when opening the file. I think other Unix like Solaris also can do
this somehow.
Of course, this type of transfer has some limitations like locking the user
space
memory, transfer of an integer number of blocks (512 bytes), etc. But this
is not a problem.
I've found the solution in the raw I/O patch being developed by Stephen Tweddie,
but it has still some problems to be solved. I think this should be considered
to
be included in the kernel.
Bye,
Mateu
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