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Old 11-18-2010, 11:21 AM   #1
Pilot3514
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Question RAID - Questions


I am looking to set up a file server. I like the redundancy of RAID 1 and RAID 5. So here is what I am thinking. A 1TB drive is getting cheap so I thought I would start with 3 - 1TB drives in a RAID 5 configuration. This should give me 2TB of storage.
Some assumptions:
1) I will need to keep 100M outside the RAID for /BOOT.
2) I may as well keep 100M partition on every drive so all partitions are the same size

My questions:
1) Can I create a "/boot" on every disk so I have a redundancy there as well? Cost is low.
2) After the RAID is up and running. How do I know if one of the disks is failing?
3) Can I hot swap a drive? If so how is that done?
4) Once I replace the bad drive. I assume I need to fdisk the new drive to create the 100M partition and the RAID partition but how is it then added into the existing RAID to replace the failed unit?
5) If I fill the file system and need more storage, how do I add storage? Can I add another 1TB drive to the existing 3? Can I add a drive of a different capacity?
6) Will the Kernel insure that the data is correctly spread across the disks? It would suck to have both a data segment and it's parity on the same disk.

I hope this question is appropriate for this forum.

Last edited by Pilot3514; 11-18-2010 at 11:24 AM. Reason: Change T to TB for TeraByte
 
Old 11-18-2010, 11:48 AM   #2
AlucardZero
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Hardware or software RAID? A hardware RAID controller is highly recommended due to processing and administration benefits.

Also, I could answer most of your questions if it's hardware, but not if it's software
 
Old 11-18-2010, 06:27 PM   #3
jf.argentino
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Quote:
A hardware RAID controller is highly recommended due to processing and administration benefits.
IMHO this is the software RAID that is highly recommended for (at least) following reasons:
-price: soft RAID cost nothing, true RAID controller generally cost many bucks. Fake hardware RAID, done by BIOS are found on high end computer, have no really performance gain over software RAID (if they are supported by linux at all), and have the same constraint than the real hardware RAID (see my second point)
-soft RAID can easily aggregate a whole hard drive with a subpartition of another one, hardware RAID needs same disk sizes for all disk in the same array.
Quote:
1) Can I create a "/boot" on every disk so I have a redundancy there as well? Cost is low.
I think the redundancy for boot partition is a waste since grub, afaik, can not handle software (and hardware?) raid, so if your /boot partition is broken, your system won't boot.
Quote:
2) After the RAID is up and running. How do I know if one of the disks is failing?
mdadm will report any problem into log file, so as any other admin task, you scan log files for problem, or let a script doing this for you and send email.
Quote:
3) Can I hot swap a drive? If so how is that done?
I think that sata drive can be hot-swapped, and by using an harddrive drawer you haven't to open the system while it is running
Quote:
4) Once I replace the bad drive. I assume I need to fdisk the new drive to create the 100M partition and the RAID partition but how is it then added into the existing RAID to replace the failed unit?
it's as easy as something like "mdadm add NEW_PARTITION /dev/md0" where /dev/md0 is your raid array, then mdadm will copy the array content into the new partition.
Quote:
6) Will the Kernel insure that the data is correctly spread across the disks? It would suck to have both a data segment and it's parity on the same disk.
Don't worry, developers are smart enough... RAID1 save my life once so it has work at least once... More seriously mdadm is really mature now and used in many production environment.

Last edited by jf.argentino; 11-18-2010 at 06:28 PM. Reason: precisions
 
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Old 11-18-2010, 06:43 PM   #4
AlucardZero
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Hardware RAID is transparent to the OS (unless you install the drivers & tools to read the RAID card). grub doesn't care that the disk it see is really a RAID array and will happily boot off of it.
 
Old 11-19-2010, 03:42 AM   #5
jf.argentino
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Quote:
grub doesn't care that the disk it see is really a RAID array and will happily boot off of it.
With the performance, I think this is the only real benefit of true hardware RAID: you can RAID the whole system, not just data.

here's a comparison between both solutions, there's one point where I'm not agree it's for the hotswap, I have a hd drawer and a spare disk, i'll try this asap, and then I would be able to answer to your question 5.
 
Old 11-19-2010, 04:12 AM   #6
batfastad
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I've been using software RAID 10 in production for about 6 months and so far no problems. I've even got a hot-spare drive configured in case one drive in the array fails.

I believe that with modern hardware, the relatively lower performance of software RAID vs hardware is no longer an issue.

The only time I would go with hardware RAID would be for something like RAID 6, probably RAID 5 as well. Those RAID levels require parity calculations to be performed which would in theory put extra load on the processor with software RAID. A hardware RAID card deals with all that internally.
I don't believe the increased processor loads for software RAID levels 1/0/10 is anything to worry about these days.

Chances are the bottleneck in your storage will actually be your slow SATA drives (vs enterprise SAS drives). Not the extra processor load required for software RAID.

Hope this helps, anyone feel free to correct me if I'm wrong

Cheers, B
 
Old 11-19-2010, 04:29 AM   #7
jf.argentino
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I'm coming back...
So hot swapping is straightforward with SATA hd in trays. I just add a new hard-drive to my raid1 while it is running, the new disk is marked as "spare" and the array will automatically resync it when one of the other drives will fail.
It looks like increasing the array size should be doable without rebooting the system, nor un-mounting the array (at least in RAID1) by changing each array drive one at a time for bigger model (between each change you have to wait the array is fully resynchronized), then resize2fs, but I don't have tested this. For sure it is doable if you can afford a reboot / an unmounting of you partition.
 
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Old 11-19-2010, 01:34 PM   #8
Pilot3514
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It looks like mdadm is the program I need to look at.
I assume that
Quote:
mdadm add NEW_PARTITION /dev/md0
means something like:
mdadm add /dev/sdc1 /dev/md0/
to add sdc1 to the RAID md0.

I further assume that there is some way for me to tell the RAID with this admin tool to use the disk or hold it as a hot spare.

So it looks like I can keep adding disks as needed until I run out of controllers.

I will need to read up on this command mdadm.

Thanks so much.
 
  


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