Re: Raw LVM devices on Linux

The main concern about raw devices vs cooked files is supposed to be performance, or precisely - KAIO. Try "onstat -g ath | grep kio" to see if kaio is on. If it is then don't worry anymore. If it isn't then I suppose you should do some more research in the same direction. HTH "Jeff" <nobody@localhost> wrote in message news:441750ae$1@news.mcleodusa.net... > I'm having a difficult time finding a definitive answer to the proper > usage of raw devices on Linux. We are running IDS 9.40 on SLES9 using LVM > partitions for our dbspaces. We use soft links to the > /dev/volgroup/volname files. While this clearly is not "cooked" file > system space, is it buffered by the OS? The /dev/volgroup/volname devices > are block devices, not character devices:

I see now that the machine notes for release 9.40UC7 say that Kernel AIO is NOT supported. So does that make the difference between block and character devices irrelevant? Or might there still be some advantage (i.e., no buffering) to using /dev/raw vs. /dev/volgroup/volname? -- Jeff jlar310 at yahoo Davorin Kremenjas wrote: > The main concern about raw devices vs cooked files is supposed to be > performance, or precisely - KAIO. > Try "onstat -g ath | grep kio" to see if kaio is on. > If it is then don't worry anymore. > If it isn't then I suppose you should do some more research in the same > direction. > > HTH > > "Jeff" <nobody@localhost> wrote in message > news:441750ae$1@news.mcleodusa.net... >> I'm having a difficult time finding a definitive answer to the proper >> usage of raw devices on Linux. We are running IDS 9.40 on SLES9 using LVM >> partitions for our dbspaces. We use soft links to the >> /dev/volgroup/volname files. While this clearly is not "cooked" file >> system space, is it buffered by the OS? The /dev/volgroup/volname devices >> are block devices, not character devices: > >

Jeff wrote: > I see now that the machine notes for release 9.40UC7 say that Kernel AIO > is NOT supported. So does that make the difference between block and > character devices irrelevant? Or might there still be some advantage > (i.e., no buffering) to using /dev/raw vs. /dev/volgroup/volname? Initialising raw devices and storing your data in the raw devices (or soft links pointing to them) is definitely faster on Linux even without KAIO than using cooked files. I see you're trying to point to the logical volume directly which is not something I have tried but I strongly suspect it is not the same as initialising raw devices and using them. When you use raw you get entries in /proc/partitions and I am sure that there is kernel code for allowing raw devices. You need version 10.00 for a KAIO implementation on Linux. On 9.40 I still recommend you use raw even without KAIO. It's not as if it takes much effort to set it up ;-) Ben.