Messages in this thread | | | Subject | Re: performance of O_DIRECT on md/lvm | From | Andi Kleen <> | Date | 20 Jan 2002 23:11:35 +0100 |
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arjan@fenrus.demon.nl writes:
> In article <p734rlg90ga.fsf@oldwotan.suse.de> you wrote: > > > I think an optional readahead mode for O_DIRECT would be useful. > > I disagree. O_DIRECT says "do not cache. period. I know what I'm doing" > and the kernel should respect that imho. After all we have sys_readahead for > the other part...
Problem with sys_readahead is that it doesn't work for big IO sizes. e.g. you read in big blocks. You have to do readahead(next block); read(directfd, ..., big-block); The readahead comes to early in this case; it would be better if it is done in the middle of read of big-block based on the request size. Otherwise you risk additional seeks when you overflow the 'read window', which is all to easy this way.
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