Real async file IO on Linux?

[Mingfai]

Using buffered IO this is hard to implement scalably. A naive implementation
> will write data as it arrives (into the OS buffer), and then call sync (or
> fsync or whatever). When that call has returned it sends its response back
> to the client saying "data persisted ok". Problem is this doesn't scale when
> you have many (possibly many thousands) of connections calling sync for each
> write. Sync is a heavyweight operation.
>
> This can be worked around to some extent, by "batching" of fsync calls.
> E.g. you only sync at most, say, every 10 milliseconds, after which all
> writes waiting for a sync can return their completion. The problem with this
> is introduces extra latency to completion for each client connection. It
> also fairly tricky to code.
>
>
how about a server system has a RAID controller card with Battery/Flash
Backed Write Cache? your "doesn't scale well" statement is true in the case
without BBWC, but if BBWC is available, it seems to me it is desirable to
keep sending fsync (instead of in batch), and let the RAID controller card
to take care of batching. AFAIK, all db server shall have that piece of
hardware and that made a huge difference in io performance. *[1] what do you
think?

regards,
mingfai

*[1] an example of benchmark with/without write cache
http://blog.a2o.si/2009/06/19/hp-dl380-g5-drive-write-cache-bbwc/
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