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SubjectRe: [blockdevices/NBD] huge read/write-operations are splitted by the kernel
On Mon, Sep 08 2003, Sven Köhler wrote:
> Hi,
>
> i discussed a problem of the NBD-protocl with Pavel Machek. The problem
> i saw is that there is no maximum for the length field in the requests
> that the NBD kernel module sends to the NBD server. Well, this length
> field is the length field from the read/write-operation that the kernel
> delegates to the blockdevice-implementation.
> I did some tests tests like
> dd if=dev/nbd/0 of=/dev/null bs=10M
> and our NBD-server implementation printed out the length field of each
> reqeust. There was a very regular pattern like
> 0x1fc00 (127KB)
> 0x00400 (1KB)
> 0x1fc00
> 0x00400
> ...
> Well, can anybody explain that to me?
> (why so "little" 1KB requests? but that's not important)
>
> Well, i also tested
> dd if=dev/nbd/0 of=/dev/null bs=1
> which means that the device will be read in chunks of 1byte.
> The result was the same: 127KB, 1KB, 127KB, 1KB...
>
> I guess the caching layer is inbetween, and will devide the huge 10MB
> requests into smaller 127KB ones, as well as joining the small 1byte
> requests by using read-ahead i guess.
> Perhaps you could tell me how i can turn off caching. Than i will test
> again without the cache.
>
> The thing i want to know is, if there is any part of the kernel that
> gaarantees that a read/write requests will not be bigger that a certain
> value. If there is no such upper limit, the NBD itself would need to
> split things up which might become a complicated task. This task need to
> be done, because it can become very difficult for the NBD server to
> handle huge values, and one huge requests will block all other pending
> small ones due to limitations of the NBD protocol.

You'll probably find that if you bump the max_sectors count if your
drive to 256 from 255 (that is the default if you haven't set it), then
you'll see 128kb chunks all the time.

See max_sectors[] array.

--
Jens Axboe

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