[Stgt-devel] User-mode iSER

[FUJITA Tomonori]

From: "Dan Bar Dov" <danb at voltaire.com>
Subject: Re: [Stgt-devel] User-mode iSER
Date: Thu, 3 Aug 2006 11:34:33 +0300

>  
> 
> > -----Original Message-----
> > From: stgt-devel-bounces at lists.berlios.de 
> > [mailto:stgt-devel-bounces at lists.berlios.de] On Behalf Of Ming Zhang
> > Sent: Wednesday, August 02, 2006 5:20 PM
> > To: Tom Tucker
> > Cc: FUJITA Tomonori; stgt-devel at lists.berlios.de
> > Subject: Re: [Stgt-devel] User-mode iSER
> > 
> > On Wed, 2006-08-02 at 09:01 -0500, Tom Tucker wrote:
> > > [...snip...]
> > > > > 
> > > > > I think this is the area where we will need to get 
> > fancy if we want
> > > > > higher performance. To avoid the copy, we would have to 
> > migrate to
> > > > > netchannels (if they every happen) or implement our own 
> > simple tear-away
> > > > > buffer scheme on top of a socket. I think this is 
> > phase-ii, however. 
> > > > 
> > > > ok, otherwise copy to user space and copy back to kernel 
> > for disk = low
> > > > performance. yes, direct io can be used here, but then u 
> > lose whole
> > > > cache benefits.
> > > 
> > > Can you elaborate on the loss of "whole cache benefits". 
> > 
> > if you use a Linux box as a storage server, it will be desired to use
> > page cache as storage cache. though this will bring data integrity
> > issues, but so many people still want to have it for specific
> > applications.
> > 
> 
> I don't think page cache as storage cache makes sense. If you look
> at the networked storage, you have caching on the initiator (client) side
> and you have caching on the target (storage server) side. Storage
> servers usually have write-behind cache, and the better ones, have
> it battery backed up for data integrity's sake. Storage admins know to
> shutd down write behind caching if it is not backed up.

Modern operating systems and applications (like file systems) does not
need help from battery-backed memory to enjoy write-behind cache on
SAN target devices for better performance without data corruption
risks. So page cache is always useful.

Of cource, we can implement own read-ahead or more clever cache
algorithms, however we can use page cache for free.