[Iscsitarget-devel] Re: [Scst-devel] Re: Ang: Re: [Stgt-devel] Re: [Iscsitarget-devel] stgt a new version of iscsi target?
vst at vlnb.net
Wed Dec 14 16:17:07 CET 2005
FUJITA Tomonori wrote:
> From: Vladislav Bolkhovitin <vst at vlnb.net>
> 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.
Are you sure? Why do you think using the cache together with its nice
features like read-ahead doesn't bring major performance benefits?
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