On Thu, April 7, 2011 7:31 pm, BRM wrote: > ----- Original Message ---- > >> From: Joost Roeleveld <jo...@antarean.org> >> On Thursday 07 April 2011 06:52:26 BRM wrote: >> > ----- Original Message ---- >> > >> > > From: Joost Roeleveld <jo...@antarean.org> >> > > >> > > On Thursday 07 April 2011 06:20:55 BRM wrote: >> > > > ----- Original Message ---- >> > > > >> > > > > From: Neil Bothwick <n...@digimed.co.uk> >> > > > > >> > > > > On Thu, 07 Apr 2011 05:22:41 -0500, Dale wrote: >> > > > > > I want to do it this way because I don't trust LVM enough >> > > > > > to put my >> > > > > > >> > > > > > OS >> > > > > > on. Just my personal opinion on LVM. >> > > > > >> > > > > This doesn't make sense. Your OS can be reinstalled in an >> hour >> > > > > or two, your photos etc. are irreplaceable. >> > > > >> > > > Makes perfect sense to me as well. >> > > > >> > > > Having installed LVM - and then removed it due to issues; namely, >> > > > the fact that one of the hard drives died taking out the whole >> LVM >> > > > group, leaving the OS unbootable, and not easily fixable. There >> > > > was a thread on that (started by me) a while back (over a >> year). >> > > > >> > > > So, perhaps if I had a RAID to underly so I could mirror drives >> > > > under LVM >> > > > >> > > > for recovery I'd move to it again. But otherwise it is just a >> PITA >> > > > waiting >> > > > >> > > > to happen. >> > > > >> > > > Ben >> > > >> > > Unfortunately, any method that spreads a filesystem over multiple >> disks >> > > can be >> > > >> > > affected if one of those disks dies unless there is some mechanism >> in >> > > place that can handle the loss of a disk. >> > > For that, RAID (with the exception of striping, eg. RAID-0) >> provides >> > > that. >> > > >> > > Just out of curiousity, as I never had the need to look into this, >> I >> > > think that, in theory, it should be possible to recover data from >> LVs >> > > that were not >> > > >> > > using the failed drive. Is this assumption correct or wrong? >> > >> > If you have the LV configuration information, then yes. Since I >> managed to >> > find the configuration information, I was able to remove the affected >> PVs >> > from the VG, and get it back up. >> > I might still have it running, but I'll back it out on the next >> rebuild - > or >> > if I have a drive large enough to do so with in the future. I was >> wanting >> > to use LVM as a bit of a software RAID, but never quite got >> > that far in the configuration before it failed. It does do a good job >> at >> > what it's designed for, but I would not trust the OS to it either >> since the >> > LVM configuration is very important to keep around. >> > >> > If not, good luck as far as I can tell. >> > >> > Ben >> >> LVM isn't actually RAID. Not in the sense that one gets redundancy. If >> you >> consider it to be a flexible partitioning method, that can span >> multiple >>disks, >> >> then yes. >> But when spanning multiple disks, it will simply act like JBOD or >> RAID0. >> Neither protects someone from a single disk failure. >> >> On critical systems, I tend to use: >> DISK <-> RAID <-> LVM <-> Filesystem >> >> The disks are as reliable as Google says they are. They fail or they >> don't. >> RAID protects against single disk-failure >> LVM makes the partitioning flexible >> Filesystems are picked depending on what I use the partition for >> > > The attraction to LVM for me was that from what I could tell it supported > and > implemented a software-RAID > so that I could help protect from disk-failure. I never got around to > configuring that side of it, but that was my goal. > Or are you saying I was misunderstanding and LVM _does not_ contain > software-RAID support?
Unless I am mistaken, LVM does not provide redundancy. It provides disk-spanning (JBOD) and basic striping (RAID-0). For redundancy, I would use a proper RAID (either hardware or software). On top of this, you can then decide to have a single filesystem, LVM or even partition this. I think the confusion might have come from the fact that both LVM and Linux Software Raid use the "Device Mapper" interface in the kernel config and they are in the same part. Also, part of the problem is that striping is also called RAID-0. That, to people who don't fully understand it yet, makes it sound like it is a RAID. It actually isn't as it doesn't provide any redundancy. I do hope you didn't loose too much important data when you had this issue. -- Joost
