On Tue, 04 Nov 2008 17:32:51 +0100, Scott Long <[EMAIL PROTECTED]> wrote:
Patrick M. Hausen wrote:
h, all,
On Tue, Nov 04, 2008 at 03:32:01PM +0100, d_elbracht wrote:
Hi list,
can someone please explain, why stat -x /dev/da1
show the SCSI-Drive as a character-device ?
http://www.freebsd.org/doc/en/books/arch-handbook/driverbasics-block.html
Wow that's a confusing and misleading article.
1. disk access in the driver layer still happens on a block basis. It's
true that to the application layer, the device has character dev
semantics, meaning that arbitrary numbers of bytes can be accessed
randomly without any restrictions. But deep down inside the kernel,
it's still doing block-by-block access.
2. caching still happens at the filesystem level. Doing I/O directly to
/dev/daX or adX or whatever will not be cached/buffered, but doing I/O
to a file on any of these devices will.
3. Cache coherency between the block and character device
representations was indeed an issue, but removing the block/cached
representation was really a matter of policy over tools, and it's
one reason why FreeBSD gets creamed in the silly-io-benchmarch
department.
4. However, in the not-so-silly-io-benchmark department, I think FreeBSD
does a whole lot better because you don't have the blind caching of the
block device trying to out-guess what the filesystem is trying to do.
Scott
This explains some things to me as a simple user reading the linked
article.
I'm not a kernel programmer, but do understand computers and this article
made me wonder if I missed something last years about disks and caches.
Ronald.
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