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    SubjectRe: Integration of SCST in the mainstream Linux kernel
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    On Mon, 2008-02-04 at 20:56 +0300, Vladislav Bolkhovitin wrote:
    > James Bottomley wrote:
    > > On Mon, 2008-02-04 at 20:16 +0300, Vladislav Bolkhovitin wrote:
    > >
    > >>James Bottomley wrote:
    > >>
    > >>>>>>So, James, what is your opinion on the above? Or the overall SCSI target
    > >>>>>>project simplicity doesn't matter much for you and you think it's fine
    > >>>>>>to duplicate Linux page cache in the user space to keep the in-kernel
    > >>>>>>part of the project as small as possible?
    > >>>>>
    > >>>>>
    > >>>>>The answers were pretty much contained here
    > >>>>>
    > >>>>>http://marc.info/?l=linux-scsi&m=120164008302435
    > >>>>>
    > >>>>>and here:
    > >>>>>
    > >>>>>http://marc.info/?l=linux-scsi&m=120171067107293
    > >>>>>
    > >>>>>Weren't they?
    > >>>>
    > >>>>No, sorry, it doesn't look so for me. They are about performance, but
    > >>>>I'm asking about the overall project's architecture, namely about one
    > >>>>part of it: simplicity. Particularly, what do you think about
    > >>>>duplicating Linux page cache in the user space to have zero-copy cached
    > >>>>I/O? Or can you suggest another architectural solution for that problem
    > >>>>in the STGT's approach?
    > >>>
    > >>>
    > >>>Isn't that an advantage of a user space solution? It simply uses the
    > >>>backing store of whatever device supplies the data. That means it takes
    > >>>advantage of the existing mechanisms for caching.
    > >>
    > >>No, please reread this thread, especially this message:
    > >>http://marc.info/?l=linux-kernel&m=120169189504361&w=2. This is one of
    > >>the advantages of the kernel space implementation. The user space
    > >>implementation has to have data copied between the cache and user space
    > >>buffer, but the kernel space one can use pages in the cache directly,
    > >>without extra copy.
    > >
    > >
    > > Well, you've said it thrice (the bellman cried) but that doesn't make it
    > > true.
    > >
    > > The way a user space solution should work is to schedule mmapped I/O
    > > from the backing store and then send this mmapped region off for target
    > > I/O. For reads, the page gather will ensure that the pages are up to
    > > date from the backing store to the cache before sending the I/O out.
    > > For writes, You actually have to do a msync on the region to get the
    > > data secured to the backing store.
    >
    > James, have you checked how fast is mmaped I/O if work size > size of
    > RAM? It's several times slower comparing to buffered I/O. It was many
    > times discussed in LKML and, seems, VM people consider it unavoidable.

    Erm, but if you're using the case of work size > size of RAM, you'll
    find buffered I/O won't help because you don't have the memory for
    buffers either.

    > So, using mmaped IO isn't an option for high performance. Plus, mmaped
    > IO isn't an option for high reliability requirements, since it doesn't
    > provide a practical way to handle I/O errors.

    I think you'll find it does ... the page gather returns -EFAULT if
    there's an I/O error in the gathered region. msync does something
    similar if there's a write failure.

    > > You also have to pull tricks with
    > > the mmap region in the case of writes to prevent useless data being read
    > > in from the backing store.
    >
    > Can you be more exact and specify what kind of tricks should be done for
    > that?

    Actually, just avoid touching it seems to do the trick with a recent
    kernel.

    James




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