Messages in this thread | | | Date | Sun, 13 Dec 1998 16:41:05 +0000 | From | Jan-Simon Pendry <> | Subject | Re: PATCH: Raw device IO for 2.1.131 |
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Gerard Roudier wrote: > > On Sat, 12 Dec 1998, Linus Torvalds wrote: > > > So I want to be able to do things like frame capture (into a _file_, not a > > partition) etc, and there I think something like sendfile() is the answer > > (well, "copyfd()", whatever). And there I also think that we currently > > would do fairly badly on the write-out part due to the known problems with > > buffer cache behaviour under certain circumstances. But basically I still > > see absolutely no reason for supporting raw devices per se. > > You may forget Linux/Unix O/Ses legacies for just a minute and think of > the following: > Using Direct I/Os from user-land synchronously is indeed very bad for > streaming since everything will rely on disk write caching and prefetching > ability. But by combining direct IOs with an asynchonous I/O completion > mechanism, applications are able to implement kind of read-ahead and/or > write coalescence algorithms as needed. > > Just my 0,02 Euro which is a little more than 0.02$. ;-) > > Regards, > Gerard.
indeed, this is just what databases such as oracle and informix like to do for maximum performance. remember, the oracle rdbms likes to think of itself as an operating system. it has a scheduler, a buffer cache, i/o queues, multi-threading, etc. etc. the more of the o/s it can get out of the way (eg any kind of o/s level block buffering) the better the overall system performance will be. of course, it might be better if oracle just let the o/s do all the buffering, but a) it doesn't, and b) the o/s block replacement strategy probably isn't what a database would want anyway. hmmm...
jan-simon.
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