Re: [ANNOUNCE] iSCSI enterprise target software

[Ming Zhang <mingz@ele.uri.edu>]

On Tue, 2005-03-01 at 13:37, Arjan van de Ven wrote:
> On Tue, 2005-03-01 at 10:24 -0800, Bryan Henderson wrote:
> > One thing that's implicit in your reasons for wanting to be in the kernel 
> > is that you've chosen to exploit the kernel's page cache.  As a user of 
> > the page cache, you have more control from inside the kernel than from 
> > user space.  The page cache was designed to be fundamentally invisible to 
> > user space.
> > 
> > A pure user space implementation of an ISCSI target would use process 
> > virtual memory for a cache and manage it itself.  It would access the 
> > storage with direct I/O. 
> 
> why would it use direct I/O ? Direct I/O would be really stupid for such
> a thing to use since that means there's no caching going on *at all*.
> 
what Bryan suggest is a privately owned and managed user space cache. so
for that disk write should be real write-through.

it is hard to beat linux kernel cache performance though.

> You want to *use* the kernel pagecache as much as you can. You do so by
> using mmap and such, and msync to force content to disk. That uses the
> kernel pagecache to the maximum extend, while not having to bother with
> knowing the intimate details of the implementation thereof, which a
> kernel side implementation would be involved in. (if it wasnt and only
> used highlevel functions, then you might as well do the same in
> userspace after all)
> 
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