On Tue, 2010-10-26 at 09:29 +1300, Ian Collins wrote: > On 10/26/10 08:53 AM, Edward Martinez wrote: > >> ZFS can not do it's magic with > >> error detection and correction if you use hw raid. > >> > > Thanks! > > > > I was not aware ZFS loses many of its features when used with hw RAID > > cards. In that case I'm not getting it > > > > > It doesn't loose many (or any if you just serve JBOD from the RAID > card), just one of the most important - control of redundancy! >
Well... As always, the issue is more complex. :-) Realistically, you want ZFS to handle all of the RAID configuration, to make maximum use of its features. That means, for the most part, you will bypass all the extra features of a Hardware Raid Controller, likely running it in either "JBOD Mode", or simply configuring it to export each individual disk as a Raid 0 LUN. However, there are still a couple of major features in a HW controller that you might be interested in: (a) for boot drives, ZFS (like all software RAID systems) is UNABLE to automatically (i.e. unattended) boot from the secondary mirror half if the primary boot drive fails. This is a BIOS limitation - if you use any software raid solution, and disk 0 fails, you will have to go to the BIOS itself to instruct it to boot from disk 1. (I'm slightly over-simplifying, but this is true for the vast majority of systems and HBA/Raid cards). If you use a RAID card and create a mirror, it presents this to the BIOS as a single disk, and thus, you can reboot if either dies. (b) ZFS currently supports only single vdev rpool pools, and the vdev must either be a mirror or single disk. HW controllers have no such restrictions. (c) If (and ONLY IF) you have a battery-backed NVRAM cache on the Raid Controller, this can provide a significant performance boost over a plain HBA. This cache can be used even if you don't use the HW Raid functionality. If you have a cache, but no battery, then I'd really not recommend using that kind of Raid Controller at all. Data loss looms. -- Erik Trimble Java System Support Mailstop: usca22-317 Phone: x67195 Santa Clara, CA Timezone: US/Pacific (GMT-0800) _______________________________________________ opensolaris-discuss mailing list opensolaris-discuss@opensolaris.org
