Messages in this thread | | | Date | Mon, 09 Apr 2007 23:58:55 -0400 | From | Mark Lord <> | Subject | Re: Lower HD transfer rate with NCQ enabled? |
| |
Phillip Susi wrote: > Mark Lord wrote: >> Phillip Susi wrote: >>> Sounds like this is a serious bug in the WD firmware. >> >> For personal systems, yes. For servers, probably not a bug. >> >> Disabling readahead means faster execution queued commands, >> since it doesn't have to "linger" and do unwanted read-ahead. >> So this bug is a "feature" for random access servers. >> And a big nuisance for everything else. > > I think you misunderstand the bug. The bug is not that the drive > disables internal readahead; the bug is that host supplied readahead > requests work so horribly. It is a good thing that the drive allows the > host to control the readahead, but something is wrong if the drive's > readahead is WAY better than any the host can perform.
Well, in this case, it has already been determined that switching to a different Linux I/O scheduler gives back most of the performance.
But the drive can do readahead better than the OS: With the OS, everything is broken up into discrete requests, whereas with the drive firmware, it can continuously update it's readahead projections, even in the midst of a command. So it does have an advantage.
But again, only the WD Raptor seems to have serious problems here. Other drives cope well with readahead + NCQ just fine.
Cheers - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
| |