>>>>> "Ian" == Ian Pilcher <arequipeno@xxxxxxxxx> writes: Ian> On 6/23/20 4:30 PM, John Stoffel wrote: >> Well, as you add LVM volumes to a VG, I don't honestly know offhand if >> the areas are pre-allocated, or not, I think they are pre-allocated, >> but if you add/remove/resize LVs, you can start to get fragmentation, >> which will hurt performance. Ian> LVs are pre-allocated, and they definitely can become fragmented. Ian> That's orthogonal to whether the VG is on a single RAID device or a Ian> set of smaller adjacent RAID devices. >> No, you still do not want the partitioned setup, becuase if you lose a >> disk, you want to rebuild it entirely, all at once. Personally, 5 x >> 8Tb disks setup in RAID10 with a hot spare sounds just fine to me. >> You can survive a two disk failure if it doesn't hit both halves of >> the mirror. But the hot spare should help protect you. Ian> It depends on what sort of failure you're trying to protect against. If Ian> you lose the entire disk (because of an electronic/mechanical failure, Ian> for example) your doing either an 8TB rebuild/resync or (for example) Ian> 16x 512GB rebuild/resyncs, which is effectively the same thing. Ian> OTOH, if you have a patch of sectors go bad in the partitioned case, Ian> the RAID layer is only going to automatically rebuild/resync one of the Ian> partition-based RAID devices. To my thinking, this will reduce the Ian> chance of a double-failure. Once a disk starts throwing errors like this, it's toast. Get rid of it now. Ian> I think it's important to state that this NAS is pretty actively Ian> monitored/managed. So if such a failure were to occur, I would Ian> absolutely be taking steps to retire the drive with the failed sectors. Ian> But that's something that I'd rather do manually, rather than kicking Ian> off (for example) and 8TB rebuild with a hot-spare. Sure, if you think that's going to happen when you're on vacation and out of town and the disk starts flaking out... :-) >> One thing I really like to do is mix vendors in my array, just so I >> dont' get caught by a bad batch. And the RAID10 performance advantage >> over RAID6 is big. You'd only get 8Tb (only! :-) more space, but much >> worse interactive response. Ian> Mixing vendors (or at least channels) is one of those things that I Ian> know that I should do, but I always get impatient. Ian> But do I need the better performance. Choices, choices ... :-) >> Physics sucks, don't it? :-) Ian> LOL! Indeed it does! >> What I do is have a pair of mirrored SSDs setup to cache my RAID1 >> arrays, to give me more performance. Not really sure if it's helping >> or hurting really. dm-cache isn't really great at reporting stats, >> and I never bothered to test it hard. Ian> I've played with both bcache and dm-cache, although it's been a few Ian> years. Neither one really did much for me, but that's probably because Ian> I was using write-through caching, as I didn't trust "newfangled" SSDs Ian> at the time. Sure, I understand that. It makes a difference for me when doing kernel builds... not that I regularly upgrade. >> My main box is an old AMD Phenom(tm) II X4 945 Processor, which is now >> something like 10 years old. It's fast enough for what I do. I'm >> more concerned with data loss than I am performance. Ian> Same here. I mainly want to feel comfortable that I haven't crippled my Ian> performance by doing something stupid, but as long as the NAS can stream Ian> a movie to media room it's good enough. Ian> My NAS has an Atom D2550, so it's almost certainly slower than your Ian> Phenom. Yeah, so that's another strike (possibly) against RAID6, since it will be more CPU overhead, esp if you're running VMs at the same time on there.
