On Wed, Mar 27, 2013 at 1:10 PM, Stan Hoeppner <stan@xxxxxxxxxxxxxxxxx> wrote: > On 3/27/2013 2:23 PM, Mark Knecht wrote: > >> Note that another level of understanding (which I don't have) has to >> do with getting chunk sizes that work well for my needs. That's a >> whole other kettle of fish... > ... >> mark@c2stable ~ $ cat /proc/mdstat >> Personalities : [linear] [raid0] [raid1] [raid10] [raid6] [raid5] [raid4] >> md6 : active raid5 sdc6[1] sdd6[2] sdb6[0] >> 494833664 blocks super 1.1 level 5, 64k chunk, algorithm 2 [3/3] [UUU] >> bitmap: 0/2 pages [0KB], 65536KB chunk >> >> md3 : active raid6 sdd3[2] sdc3[1] sdb3[0] sde3[3] sdf3[5] >> 157305168 blocks super 1.2 level 6, 16k chunk, algorithm 2 [5/5] [UUUUU] >> >> md7 : active raid6 sdd7[2] sdc7[1] sdb7[0] sde2[3] sdf2[4] >> 395387904 blocks super 1.2 level 6, 16k chunk, algorithm 2 [5/5] [UUUUU] >> >> md126 : active raid1 sdb5[0] sdd5[2] sdc5[1] >> 52436032 blocks [3/3] [UUU] > > Your problem isn't chunk sizes, but likely the 4 md/RAID devices atop > the same set of physical disks. If you have workloads that are > accessing these md devices concurrently that will tend to wreak havoc > WRT readahead, the elevator, and thus the disk head actuators. If these > are low RPM 'green' drives it will be exacerbated due to the slow > spindle speed. > The drives are WD RE3 so at least I have that in my favor. I started learning these lessons about multiple RAIDs on one set of physical disks after building the machine. The plan I'm moving _very_ slowly toward is migrating from the md126 RAID1 which is currently root to the md3 RAID6. I've built a new, bootable Gentoo install on the RAID6. It's up and running and basically I think I just need to move my user account and the stuff in /home and I'm there. With that done md126 is gone. md7 is manageable. It's all Virtualbox VMs which I backup externally every week so I can do a backup of that, delete md126 & md7 and then (hopefully) resize md3 larger. md6 isn't used much. I mount it, do quick backups to it, and then unmount. It's used about once a day and not in use most of the time. I could probably get rid of it completely but I'd want another external drive to replace it. Anyway, if's not overly important one way or another. All that said, I still don't really know if I was starting over today how to choose a new chunk size. That still eludes me. I've sort of decided that's one of those things that make you guys pros and me just a user. :-) Cheers, Mark > The purpose of RAID is to prevent data loss when a drive fails. The > purpose of striped RAID is to add performance atop that. Thus you > normally have one RAID per set of physical disks. The Linux md/RAID > driver allows you to stack multiple RAIDs atop one set of disks, thus > shooting yourself in the foot. Look at any hardware RAID card, SAN > controller, etc, and none of them allow this--only one RAID per disk set. > > At this point you obviously don't won't to blow away your current setup, > create one array and restore, as you probably don't have backups. > Reshaping with different chunk sizes won't gain you anything either. So > about the only things you can optimize at this point are your elevator > and disk settings such as nr_requests and read_ahead_kb. Switching from > CFQ to deadline could help quite a lot. > > -- > Stan > -- To unsubscribe from this list: send the line "unsubscribe linux-raid" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
