Re: [Aoetools-discuss] write buffering and the AoE kernel module

[Jon N. <jne...@ja...>]

On Thu, 26 Apr 2007, Sam Hopkins wrote:

> Hi Jon,
> 
> > Thanks for the answer! However, it doesn't /quite/ answer my question. I 
> > should have been clearer. I know all about O_SYNC (although your 
> > description is probably the most concise I've ever seen!).  What I want 
> > to know is what (if any) caveats AoE introduces. Does the aoe driver 
> > itself buffer anything before constructing a packet and sending it off? 
> > Let me rephrase:  Does the AoE driver attempt any sort of write 
> > buffering or does the write system call translate directly into packets 
> > sent and received? If so, then I think I've got my answer.
> 
> The block layer gives the aoe driver requests and the aoe driver translates
> them into aoe read/write commands.  There's no buffering going on in
> the aoe driver.
> 
> >> Those block devices just pass block I/O through to the lower level
> >> block device driver, so they don't really affect the situation.
> > 
> > What I'm wondering, then, is why creating a single-disk raid1 (for 
> > example) changes the payload sizes of the packets going over the wire, 
> > such as when using dd to access the device directly.
> 
> Ah. That's an implementation detail of the linux block layer.  It's
> been a while since I've looked at it so my memory may have developed
> barnacles, but here's my best recollection.
> 
> When you submit I/O to a block device the block layer enforces size
> constraints on the I/O request based on the underlying size of the
> device.  If, e.g., the underlying device has an odd number of sectors,
> then the block layer is going to force the requests given to the aoe
> driver to be limited to 512 bytes.  If, e.g., the block device has an
> odd KiB in size (5KiB), the block layer would limit each I/O request to
> 1KiB.

So it is entirely a function of the /number/ of blocks (you said 
sectors, did you mean blocks?)? If I use 'stat -f' on an aoe device and 
on an md device made with that aoe device, I get 4096b blocks in both 
cases. Or am I looking at the wrong thing?

> I hope that description isn't too muddled.  I haven't had much coffee
> yet this morning.  :)

No, it's great!

--
Jon Nelson <jne...@ja...>