----- Original Message ----- From: "Neil Brown" <neilb@xxxxxxx> To: "JaniD++" <djani22@xxxxxxxxxxxxx> Cc: "Al Boldi" <a1426z@xxxxxxxxx>; <linux-raid@xxxxxxxxxxxxxxx> Sent: Wednesday, December 21, 2005 2:40 AM Subject: Re: RAID0 performance question > On Sunday December 18, djani22@xxxxxxxxxxxxx wrote: > > > > The raid (md) device why dont have scheduler in sysfs? > > And if it have scheduler, where can i tune it? > > raid0 doesn't do any scheduling. > All it does is take requests from the filesystem, decide which device > they should go do (possibly splitting them if needed) and forwarding > them on to the device. That is all. > > > The raid0 can handle multiple requests at one time? > > Yes. But raid0 doesn't exactly 'handle' requests. It 'directs' > requests for other devices to 'handle'. > > > > > For me, the performance bottleneck is cleanly about RAID0 layer used exactly > > as "concentrator" to join the 4x2TB to 1x8TB. > > But it is only a software, and i cant beleave it is unfixable, or > > tunable. > > There is really nothing to tune apart from chunksize. > > You can tune the way the filesystem/vm accesses the device by setting > readahead (readahead on component devices of a raid0 has exactly 0 > effect). First i want to sorry, about "Neil not interested" thing in previous mail... :-( I have already try the all available options, including readahead in all layer (result in earlyer mails), and chunksize. But with this settings, i cannot workaround this. And the result is incomprehensible for me! The raid0 performance is not equal with one component , with sum of all component , and not equal with the slowest component! > > You can tune the underlying devices by choosing a scheduler (for a > disk drive) or a packet size (for over-the-network devices) or > whatever. The NBD has a scheduler, and this is already tuned for really top performance, and for the components it is really great! :-) (I have planned to set the NBD to 4KB packets, but this is hard, becaused by my NICs are not supported the jumbo packets...) > > But there is nothing to tune in raid0. > > > Also, rather than doing measurements on the block devices (/dev/mdX) > do measurements on a filesystem created on that device. > I have often found that the filesystem goes faster than the block > device. I use XFS, and the two performance is almost equal, depends on kind of load. But in most often case, it is almost equal. Thanks, Janos > > > NeilBrown - To unsubscribe from this list: send the line "unsubscribe linux-raid" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
