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costel78
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Joined: 20 Apr 2007
Posts: 189

PostPosted: Sun Nov 03, 2013 10:06 pm    Post subject: To RAID5 or not ? Reply with quote

I have a almost full RAID1 array, photos, documents, backups and other files. Also want to increase the storage space by buying another HDD at same capacity like other two. Read and write speed does not really matter for me, unless they are very slow (under 30-40MB/sec).
There is two solutions:
1. Keep the raid array and simple add the new hdd to system - simple, but no safe measures regarding data on the new hdd, if it fail, my files on it are lost.
2. Create a new RAID5 array by remove one hdd from raid1 array, create a degraded raid5 array with only two disks (removed one and the new one), copy data on raid5 array from raid1 array and, finally, add the remaining raid1 disk to raid5 array.

The CPU is a slow one - AMD E-350, I think it's powerful enough for the task - simple xor operations. The system won't work 24/7, but 8-10 hours per day. Right now, I really can't afford two HDD.

The question(s):
1. Are there any precautions, other than a backup which I already have ?
2. Worth the trouble ? Should I go for raid5 or keep raid1 and add the new disk as standalone ?
3. As far I read mdadm --grow --level5 does not work if metadata on the raid1 is 1.2. True or false ?
4. With raid5 should I worry about underpowered CPU ?
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NeddySeagoon
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Joined: 05 Jul 2003
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PostPosted: Mon Nov 04, 2013 8:38 pm    Post subject: Reply with quote

ostel78,

I'm fairly sure mdadm can reshape a raid1 to raid5.
You can even use the raid while the reshape is in progress.

raid is not a substitute for backups, so as long as you have a validated backup, you should be all set.
It might be a good idea to run
Code:
echo repair  > /sys/block/md0/md/sync_action
or whatever your mdX is before you reshape the raid.
This will copy the data in any unreadable blocks on one drive from the good remaining copy.
It may force a sector reallocation when/if a copy write happens, so comparing the reallocated sector count before and after the repair may be useful too.
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NeddySeagoon

Computer users fall into two groups:-
those that do backups
those that have never had a hard drive fail.
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costel78
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Joined: 20 Apr 2007
Posts: 189

PostPosted: Mon Nov 04, 2013 8:58 pm    Post subject: Reply with quote

Well I tried in a virtual machine to change from raid1 to raid5 and it wasn't work for 1.2 metadata. "Could not set level to raid 5" was all I got despite any method.
Tried with two disk in raid1, two disk and a spare, three disk in raid1 but it wasn't work. I'm still looking for explanation or proper documentation.

Disk for backup belong to a friend, I can keep it ~ one more week and your advice it's very good - raid is not a substitute for backups. It occurred to me that I also can made a raid0 with two existing disk and buy a new drive of greater capacity to regular backups and keep it safe, out of computer.
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