[TUHS] FreeBSD disk devices (Re: Choice of Unix for 11/03 and 11/23+ Systems

[Warner Losh]

On Mon, Oct 12, 2020 at 11:47 AM Bakul Shah <bakul at iitbombay.org> wrote:

> On Oct 12, 2020, at 9:57 AM, Arthur Krewat <krewat at kilonet.net> wrote:
> >
> > On 10/11/2020 9:56 PM, Warner Losh wrote:
> >> unaligned I/O on the block device
> > Sorry, I have to laugh... isn't that an oxymoron? ;)
>
> Actually there are no *block* devices in FreeBSD.
> And while raw device files such as /dev/r<blkdev>
> no longer exist, it is the block devices (with
> their buffering etc.) that are gone. Only raw devices
> exist now.
>

There's still the buffer cache that does do buffering on the devices... but
it's a different kind of buffering than traditional kernels.  but you're
right, only the cdevs are plumbed down, not the bdevs.


> And FreeBSD raw disk device drivers don't allow
> unaligned I/O access. Nor should they paper over
> what is not allowed by the underlying device. Try
>
> #include <stdio.h>
> #include <stdlib.h>
> #include <unistd.h>
>
> unsigned char buf[1024];
>
> int main(int c, char**v) {
>         char* a = c > 1? v[1] : "1";
>         off_t off = (off_t)strtoull(a, 0, 0);
>         ssize_t r = pread(0, buf, sizeof buf, off);
>         if (r < 0) { perror("read"); return 1; }
>         for (int i = 0; i < 4; i++) printf(" %02x", buf[i]);
>         printf("\n");
>         return 0;
> }
>

I meant something different about 'unaligned' access. I mean that the
buffers you read the data into needn't be page aligned. That causes a fair
amount of hair in the lower layers where I spend much of my time...

You are correct you have to read them on an LBA boundary.

Warner
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