On Fri, 2008-09-19 at 22:39 -0500, Jonathan Brassow wrote: > Just to answer some questions: > > You can use just 2 disks to create a mirror and a disk log: > prompt> lvcreate --alloc anywhere -m1 -L 500M -n my_lv my_vg > I wish the allocation policy 'normal' (which is the default) would > also do this... but instead you must choose a looser allocation policy > to get what you want. With vg_test already containing 2 equally sized PVs (one on /dev/sda, the other on /dev/sdb), I did this: vgextend vg_test /dev/sda6 lvconvert -m 1 vg_test/XP (XP is a small LV with an XP image, /dev/sda6 is a small partition) That seems to work fine: lvs -a -o +devices XP vg_test mwi-ao 15.00G XP_mlog 100.00 XP_mimage_0(0),XP_mimage_1(0) [XP_mimage_0] vg_test iwi-ao 15.00G /dev/sda4(2560) [XP_mimage_1] vg_test iwi-ao 15.00G /dev/sdb3(0) [XP_mlog] vg_test lwi-ao 4.00M /dev/sda6(0) > The reason that you want the disk-based log is to prevent all the > extra work when you machine comes up. Yes, I understand that now. It wasn't clear from the documentation that with corelog, it would rebuild the whole mirror. > The reason that LVM wants a separate disk for the log is so the head > on the storage device doesn't need to go ping-ponging around the > device. (The log is touched for almost every write.) So, its a > performance thing... and possibly a not-wear-out-your-hardware thing > too. I see. So I guess that means that in the general case, an LVM-mirror approach is not a good substitute for an LVM-on-RAID1 approach? My original rationale was to have a more flexible system by using LVM-mirror than I'd get when using LVM-on-RAID1. What I'd expect is some way to have the log lazily cached in memory. In that way, there is no ping-ponging, you'd only have to rebuild the log/mirror when there is a power outage and you wouldn't have the startup delays, i.e. something like a regular filesystem. > It won't matter if the master disk with the log dies. Your mirror > will simply become a linear device composed of the last remaining disk > anyway. That makes sense, at least with two drives. Thanks for your comments! Koen _______________________________________________ linux-lvm mailing list linux-lvm@redhat.com https://www.redhat.com/mailman/listinfo/linux-lvm read the LVM HOW-TO at http://tldp.org/HOWTO/LVM-HOWTO/
