[Stgt-devel] User-mode iSER

[Dan Bar Dov]

> -----Original Message-----
> From: stgt-devel-bounces at lists.berlios.de 
> [mailto:stgt-devel-bounces at lists.berlios.de] On Behalf Of 
> FUJITA Tomonori
> Sent: Thursday, August 03, 2006 6:20 PM
> To: Dan Bar Dov
> Cc: mingz at ele.uri.edu; fujita.tomonori at lab.ntt.co.jp; 
> stgt-devel at lists.berlios.de; tom at opengridcomputing.com
> Subject: Re: [Stgt-devel] User-mode iSER
> 
> 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.

Since we are working on the SAN target, my point is we need to know if
this serving target is write-behind or not. If our code relies on page cache, 
it can not operate in any mode except write behind, however, this is
an implicit write behind. We could provide a faster response with an explicit 
write behind.

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