So I've learned a valuable RAID 0 lesson, and it fortunately was not a major catastrophy. I got lucky, and had a workable-enough backup on tape to make the user who needed some data happy.
Now, from the OS side, LVM is an option. Say the RAID controller only allows hardware striping or mirroring for logical volumes, but I want to use more than two disks, and I don't want the RAID 0 problem again.
When I get a replacement disk and build the system from the ground up again, I could, conceivably, use hardware RAID 1 for the OS on two disks, and CentOS 5 64-bit's LVM for software RAID 5 (or maybe 1+0 if available) on the remaining for 4 disks, maybe 3 disks as active and the 4th as a hot spare?
I've never had much faith in software raid, since it is not hardware-based, and there would be a performance hit, but in this case, it could be an option.
Insights from the OS-created RAID experience welcome.
Thanks again.
Scott
Scott R. Ehrlich wrote:
So I've learned a valuable RAID 0 lesson, and it fortunately was not a major catastrophy. I got lucky, and had a workable-enough backup on tape to make the user who needed some data happy.
Now, from the OS side, LVM is an option. Say the RAID controller only allows hardware striping or mirroring for logical volumes, but I want to use more than two disks, and I don't want the RAID 0 problem again.
Not sure what kind of RAID 0 problems you had but one option might be to run two RAID 1 mirrors on the controller, then run software RAID 0 between the two (essentially RAID 1+0). You may even be able to do native RAID 1+0 in software raid, I haven't looked into it. I haven't had a system with more than two disks that didn't have a good hardware RAID card in it.
I've never had much faith in software raid, since it is not hardware-based, and there would be a performance hit, but in this case, it could be an option.
What kind of RAID controller? In the SATA space there are plenty, I say probably most of the RAID controllers out there for SATA are shit, and I'd rather trust software raid then use them. The one exception is 3Ware. If your using SAS or parallel SCSI then typically the RAID cards there are halfway decent.
For me, if the disks are SATA, and the controller is not 3Ware then I use software RAID. If possible I go buy a 3Ware controller for the system. Software RAID works pretty well. I have it running on probably 15-20 HP DL320G4 systems (whose RAID controllers suck, we had multiple cases of drives faltering but the controller said the disks were OK, the manufacturer's diagnostics tools said the drives were going bad). On the hardware RAID front I had a couple cases where a HP SmartArray controller reported a disk was on the verge of failing but was still operational, but I/O to the array that had the faltering disk suffered significantly(75%+ drop in throughput), and the only way to get rid of the disk was to physically remove it(can be a pain when your not on site). My 3Ware controllers have a management app which you can remove disks manually from the array using the app.
If going software raid be sure your swap partition is on a RAID protected volume. There seems to be a popular (mis)conception to put swap on a RAID 0 volume or not on a RAID volume at all. If you have multiple swap partitions/files on different volumes/disks you can round robin the swap by mounting them with the same priority level, getting similar performance to RAID 0, while maintaining high(er) availability.
nate
on 3-6-2008 3:58 PM Scott R. Ehrlich spake the following:
So I've learned a valuable RAID 0 lesson, and it fortunately was not a major catastrophy. I got lucky, and had a workable-enough backup on tape to make the user who needed some data happy.
Now, from the OS side, LVM is an option. Say the RAID controller only allows hardware striping or mirroring for logical volumes, but I want to use more than two disks, and I don't want the RAID 0 problem again.
When I get a replacement disk and build the system from the ground up again, I could, conceivably, use hardware RAID 1 for the OS on two disks, and CentOS 5 64-bit's LVM for software RAID 5 (or maybe 1+0 if available) on the remaining for 4 disks, maybe 3 disks as active and the 4th as a hot spare?
I've never had much faith in software raid, since it is not hardware-based, and there would be a performance hit, but in this case, it could be an option.
Insights from the OS-created RAID experience welcome.
Thanks again.
Scott
Raid 0 is never a good option for data you need to keep. It is great if you need fast access to scratch data, though. Software raid in linux is very mature and works wonderfully, and has the benefit of being able to move all the drives to a different system and being able to start up the array without needing the controller.
On 3/7/08, Scott Silva ssilva@sgvwater.com wrote:
on 3-6-2008 3:58 PM Scott R. Ehrlich spake the following:
So I've learned a valuable RAID 0 lesson, and it fortunately was not a major catastrophy. I got lucky, and had a workable-enough backup on tape to make the user who needed some data happy.
Now, from the OS side, LVM is an option. Say the RAID controller only allows hardware striping or mirroring for logical volumes, but I want to use more than two disks, and I don't want the RAID 0 problem again.
When I get a replacement disk and build the system from the ground up again, I could, conceivably, use hardware RAID 1 for the OS on two disks, and CentOS 5 64-bit's LVM for software RAID 5 (or maybe 1+0 if available) on the remaining for 4 disks, maybe 3 disks as active and the 4th as a hot spare?
Hi there,
Minor point:
Rather than go for a RAID 5 with a hot spare you are better off going for a RAID 6 array using the 4 discs if your hardware supports it.
If your RAID 5 has a disk failure then has another whilst it is rebuilding using the hot spare then your data is b0rked whereas with RAID 6 you can tolerate 2 disk failures and still access the data.
You lose the same amount of capacity that you would have with the RAID 5 + hot spare set up that you are considering.
mike
Michael Simpson wrote:
On 3/7/08, Scott Silva ssilva@sgvwater.com wrote:
on 3-6-2008 3:58 PM Scott R. Ehrlich spake the following:
So I've learned a valuable RAID 0 lesson, and it fortunately was not a major catastrophy. I got lucky, and had a workable-enough backup on tape to make the user who needed some data happy.
Now, from the OS side, LVM is an option. Say the RAID controller only allows hardware striping or mirroring for logical volumes, but I want to use more than two disks, and I don't want the RAID 0 problem again.
When I get a replacement disk and build the system from the ground up again, I could, conceivably, use hardware RAID 1 for the OS on two disks, and CentOS 5 64-bit's LVM for software RAID 5 (or maybe 1+0 if available) on the remaining for 4 disks, maybe 3 disks as active and the 4th as a hot spare?
Hi there,
Minor point:
Rather than go for a RAID 5 with a hot spare you are better off going for a RAID 6 array using the 4 discs if your hardware supports it.
sw raid supports raid 6. Another plus for sw raid is that it's hardware agnostic - doesn't care about controller make, brand, chipset, firmware, etc.
If your RAID 5 has a disk failure then has another whilst it is rebuilding using the hot spare then your data is b0rked whereas with RAID 6 you can tolerate 2 disk failures and still access the data.
You lose the same amount of capacity that you would have with the RAID 5 + hot spare set up that you are considering.
mike
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Michael Simpson -
nate -
Scott R. Ehrlich -
Scott Silva -
Toby Bluhm