Re: Integration of SCST in the mainstream Linux kernel

From: Vladislav Bolkhovitin
Date: Mon Feb 04 2008 - 13:38:54 EST

James Bottomley wrote:
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

and here:

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: 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

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.

James, just check and you will see, buffered I/O is a lot faster.

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.

Err, to whom return? If you try to read from a mmaped page, which can't be populated due to I/O error, you will get SIGBUS or SIGSEGV, I don't remember exactly. It's quite tricky to get back to the faulted command from the signal handler.

Or do you mean mmap(MAP_POPULATE)/munmap() for each command? Do you think that such mapping/unmapping is good for performance?

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

Hmm, how can one write to an mmaped page and don't touch it?


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