Re: Ang: Re: [Stgt-devel] Re: [Iscsitarget-devel] stgt a new version of iscsi target?

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From: Vladislav Bolkhovitin <vst@xxxxxxxx>
Subject: Re: Ang: Re: [Stgt-devel] Re: [Iscsitarget-devel] stgt a new version of iscsi target?
Date: Sat, 10 Dec 2005 18:32:46 +0300

> >>>Also is the page cache comment in reference to us using the page cache 
> >>>for our reads and writes or I am not sure why you wrote that if you do 
> >>>not do it right now.
> >>
> >>Hm, at first, the page cache already used somehow in fileio dev handler 
> >>(though, with additional mem copy). At the second, fully utilize the 
> >>page cache is one of two major improvements that are pending in scst, 
> >>because it is required changing the kernel, which until some moment try 
> >>to avoid. Although I prepared what is necessary for that.
> >>
> >>The idea basically is the following. When READ operation arrives, pages 
> >>for all requested blocks are at first searched in the page cache 
> >>(probably, in SIRQ context, because it isn't expensive operation) and if 
> >>all pages are found, they are referenced and the result will be sent to 
> >>the initiator. Then the pages will be dereferenced (so, no pages 
> >>allocation will be done at all). Otherwise, the missed pages will be 
> >>allocated and the command will be rescheduled to the thread, which will 
> >>read them. Then, after the response is sent, the pages will remain in 
> >>the page cache for future accesses. For WRITEs the processing is the 
> >>similar, the pages with the data will be put in the page cache.
> > 
> > The Ardis iSCSI target code does the same thing.
> 
> Perfectly. So, why don't do it on the mid-layer level where all targets 
> can benefit from it?

Because I think that it not of much performance benefit.
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