Re: raw/block device disc troughput

[Edgar Fuß <ef%math.uni-bonn.de@localhost>]

> The block device will cause readahead at the OS layer.
But I'm writing, not reading!

> The increase is again tied to less latency in the synchronous dd read-write
> loop.  The kernel breaks the large request down to many MAXPHYS sized ones
> and dispatches each in turn.  I can't remember whether it is really
> asynchronous or whether it waits for each request to complete before issuing
> the next; if the former, it's effectively double- buffering for you.
But would you expect an eightfold increas in troughput just by that?

> What does read performance look like?  I would be particularly interested to
> know what it looks like if you use a tool like "buffer" or "ddd" that
> double-buffers the I/O for you.  It should be roughly twice the single-disk
> rate or something is wrong with RAIDframe (or, at least, suboptimal).
I will test that. I can't right now because I have no physical access atm.

On an identical machine but with the RAID's SectorsPerSU=16, I get 99MByte/s
from the raw device at 16k blocks, 191 at 64k blocks, roughly the same at
1M blocks.
On the block device, I get 19MB/s independent of block size.
All with dd.