My current setup is a mid end server (actually, low end by today's standards, 32bit CPU, non dual core) with 4 IDE drives on their own channels.
1 IDE drive is the OS drive and I did not raid it as it would probably cause complications, specially in the event of a failure. So its just seperate, and I'll just image it as a backup for the OS setup. Then theres 3 500GB drives which are in software raid5.
In linux drives show up as /dev/hd* so each individual physical drive still shows up as this is not a hardware raid so they're still treated inderpendantly. But linux also creates a raid device which shows up as /dev/md* so in my case /dev/md0
I wont go through all the commands since I'm learning myself but lets just say it was fairly easy to figure out. everything is done with the mdadm command and this is some of the outputs you can get which are quite informative:
Code: Select all
[root@alderaan vmraid1]#
[root@alderaan vmraid1]# mdadm --misc --detail /dev/md0
/dev/md0:
Version : 00.90.03
Creation Time : Sun Mar 23 19:42:07 2008
Raid Level : raid5
Array Size : 976767872 (931.52 GiB 1000.21 GB)
Device Size : 488383936 (465.76 GiB 500.11 GB)
Raid Devices : 3
Total Devices : 3
Preferred Minor : 0
Persistence : Superblock is persistent
Update Time : Mon Mar 24 21:17:33 2008
State : clean, degraded, recovering
Active Devices : 2
Working Devices : 3
Failed Devices : 0
Spare Devices : 1
Layout : left-symmetric
Chunk Size : 64K
Rebuild Status : 4% complete
UUID : b1271f7e:10899be9:8487ddbb:7b3c3fed
Events : 0.64
Number Major Minor RaidDevice State
0 22 1 0 active sync /dev/hdc1
3 3 1 1 spare rebuilding /dev/hda1
2 34 1 2 active sync /dev/hdg1
[root@alderaan vmraid1]#
[root@alderaan vmraid1]#
[root@alderaan vmraid1]#
[root@alderaan vmraid1]#
[root@alderaan vmraid1]#
[root@alderaan vmraid1]#
[root@alderaan vmraid1]# cat /proc/mdstat
Personalities : [raid6] [raid5] [raid4]
md0 : active raid5 hdg1[2] hdc1[0] hda1[3]
976767872 blocks level 5, 64k chunk, algorithm 2 [3/2] [U_U]
[=>...................] recovery = 5.0% (24702588/488383936) finish=305.4min speed=25300K/sec
unused devices: <none>
[root@alderaan vmraid1]#
[code]
Now for the fun part. To test to see how well this raid 5 works and how redundant it is, I decided to power off the server and unplug a drive. To simulate a real life situation I should really do it while its live but these are IDE drives and at hardware level they're really not meant to be hot plugged so decided not to just in case (though it would probably be no harm).
So I rebooted, at that point the raid5 still mounted ok but it was missing 1 drive, which was indicated. It also showed up as degraded. Now what would be nice is if there was some kind of daemon that would send off an email when it detects such a state, but nothing stops me from scripting something that looks at the output, or maybe there does exist something I'm not aware of.
So I made some changes to the file system, added a new file with some data in it.
Again since IDE drives are not hot swappable I powered off to plugged the 3rd drive back in. Powered on.
Server came back up, no hiccups. Now I would wish that it auto rebuilds, but guess its good that it does not since I don't want it to just go grab any random drive it finds, as that could be dangerous. So I re-add the new drive to the array. Literally a 1 command thing.
mdadm --manage -add /dev/md0 /dev/hda1
Then it says that its added, then at that point it starts rebuilding the array. This is a rather long process though, but its in the background and you can still use the array.
Now I wanted to go a step further. That server wont be on a UPS. So what if the power went out? I flicked off the power switch on the back to simulate a power outage. Waited for the drives to spin down (this server also sounds like an airplane when it powers down, so it must be really fast!). Turned the server back on. Booted right into the OS and my array still exists and continues to rebuild! So at no given time did I get stuck at some kind of bootup error where I had to physically be at the server. I'm actually quite impressed so far.
It's kind of hard to rate the speed at this point but it seems ok, not super fast but not really slow either. Took about 10 minutes to create a 10Gig VM. And raid5 is probably not the best raid type to test this with. Raid 0 would be interesting to try. My mobo supports sata so maybe in the future I'll add a couple drives to make a raid0 array.
Theres also stuff like chunk sizes to look at. For my case I should probably set my chunk size much higher as I'm dealing with larger files.
So if anyone is on a budget but wants raid, I'd defiantly take a look at mdadm command in Linux. It's a pretty nice alternative to expensive raid controllers. Also you can pretty much mix and match any kind of drive/type. I have not tried it, but you can probably even raid weird things like USB sticks, external HDDs, or maybe even flash cards hooked up to a USB flash reader. The possibilities are endless. :P From looks of it, its also possible to grow arrays. So I could probably add 2 more drives to my raid 5 to make a 1.5TB array, though not sure how it works at the partition level, guess I'd have to use a ext3 partition resizing tool that runs within the OS. Also don't think the raid array will work if you boot with a live CD so the data wont be accessable. Though if you reinstall the OS you should be able to reassemble the array.
[color=#888888][size=85]Archived topic from Iceteks, old topic ID:5020, old post ID:38726[/size][/color]
