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Re: raw/block device disc troughput
> 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
On the block device, I get 19MB/s independent of block size.
All with dd.
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