Using RAID 1 mirror for backups

Question

Level 1

0 points

Using RAID 1 mirror for backups

I am considering setting up a RAID 1 volume (1 primary and 1 secondary disk) and using the secondary mirror as the source for archive to a file-server. This should give me a perfect snapshot in time of my volume and the read access from the volume would not affect system performance too much. The strategy I have in mind is to remove the mirror from the RAID 1 volume, mount it as a normal volume and make a backup from it (using EMC Retrospect) to a file server over our network. Once the backup is done, I would unmount the mirror and reconnect it to the primary disk to recreate the RAID volume.

My questions are:

1) Is this possible with OS X's built-in RAID functionality?

2) Am I better off with using SoftRAID 3?

3) Will the rebuild of the RAID volume be as if it was from scratch (that is, would it take too long for a 150GB volume)?

4) If I have two 150GB disks (SATA), can I make the entire disk one RAID volume and still boot from it?

5) If the answer to Q4 is "no", with OS X's built-in RAID, can I make a RAID volume out of partitions from two disks? In other words, am I forced to use whole disks for a RAID members in OS X?

Your help in this would be very much appreciated.

Mac Pro Mac OS X (10.4.7)

Posted on Sep 9, 2006 4:52 AM

Reply

Answer 1

Sep 9, 2006 5:55 AM in response to Kumaradas

Hi Kumaradas;

May I welcome you to the Apple Discussion Forums.

I am only going to tackle question 5; " can I make a RAID volume out of partitions from two disks?

I am quite confident that you can use partitions of disk to create a RAID array. But, do you really want to do it is the larger question. If you do this and have the operating system in one of the other partition I think you will be see a large degradation in performance caused by head movement.

Since you have a Mac Pro which can have up to four internal disks, my solution would be to simply get a third disk. Then use in for your system and two for the RAID array.

Allan

Reply

Answer 2

besous

Level 1

120 points

Sep 9, 2006 5:56 AM in response to Kumaradas

hello kumardas

no1 : yes ...u can do that...u can un mount and remount 1 of the mirors drives
no 2 : actually iam not sure about the answer maybe i didnt get ur question v.well
no3 : yes it will take approximatly the same time (about half an hour for the 150gb)
no4 : yes u can boot from it
no5 : answe is no ... u can use 2 different partiotions from to disks to make a raid without using all the disks
have agood day

Reply

Answer 3

besous

Level 1

120 points

Sep 9, 2006 5:58 AM in response to Kumaradas

no5 : i ment yes u can make mirror with partitions....but fro different disks

Reply

Answer 4

Kumaradas Author

Level 1

0 points

Sep 9, 2006 6:50 AM in response to Allan Eckert

Thank you for the welcome. I have ordered a Mac Pro with a single 150GB drive, and I have purchased a Raptor 150G drive with the intention of using the two as I described. My preference is to have only one disk volume with the two drives as memebers of a RAID 1 set.

Q5 was relevant only if the answer to Q4 is "no". I though I read somewhere that with Intel Mac's you can not boot off a RAID volume. If you can, in fact, boot from a RAID volume (or at least boot from the primary member, and then have the RAID activated after boot), then I will use the two disks entirely for the RAID setup.

Reply

Answer 5

Kumaradas Author

Level 1

0 points

Sep 9, 2006 6:52 AM in response to besous

Thank you for your answers.

Are you sure about your answer for Q4 for Intel Mac's? I thought I have heard otherwise in some forum.

Reply

Answer 6

Kumaradas Author

Level 1

0 points

Sep 9, 2006 6:56 AM in response to besous

QRT Q2, I was referring to this product: http://www.softraid.com/vsapple.html

I'm not sure if the benefits touted by the vendor are substantial in practice.

Reply

Answer 7

Sep 9, 2006 7:05 AM in response to Kumaradas

Hi Kumaradas;

Why get a Raptor to RAID with the stock hard drive?

The RAID will only be as fast as the slowest part so using a single Raptor will buy you nothing. Generally, when set up a RAID array you strive to keep everything as nearly same as possible for best performance.

Allan

Reply

Answer 8

Kumaradas Author

Level 1

0 points

Sep 9, 2006 7:30 AM in response to Allan Eckert

I got the Raptor drive to get better performance than the stock drive (without RAID). Since then I am considereding using a mirrored RAID configuration with the Raptor and stock drive for backup snapshot and backup performance reasons.

The performance of RAID in mixed drives compared to single-drive performance was another question I had. For RAID 1 (mirrored drives) in a mixed drive configuration, I can imaging write performance being the at the speed of the slower drive (since both drives have to be written to). For reads that are striped, reads be faster than the reads from a either single drive? There would be simultaneous reads of consecutive stripes from two drives (one slow and one fast), which should be faster than sequential reads from one drive. Please correct me if I'm wrong about this.

Reply

Answer 9

The hatter

Level 9

61,031 points

Sep 9, 2006 8:54 AM in response to Kumaradas

There are a number of discussions in the Mac Pro forum on the subject of RAID, even to stripe RAID of two mirrors.

Apple RAID uses the entire drives, so while you can "short stroke" your drives, you can't mix or have multiple partitions.

There are a lot of benefits to SoftRAID but you'll have to wait if you want to boot until OS X is updated and SoftRAID is updated as well. And it will do what you want better. Though I don't understand you need to unmount to backup.

Yes, you can boot from stripped RAIDs.

Mirror helps in the event of a hardware failure. And that is all basically. Any errors are copied to both. Just having a good backup routine like you have, and it is only where you can't afford to have any down time. I keep boot RAIDs that are easy to restore and only change rarely with system updates, all the data is stored on separate drives or RAIDs of their own.

Reply

Answer 10

Kumaradas Author

Level 1

0 points

Sep 9, 2006 6:08 PM in response to The hatter

Using mirrored RAID is NOT my sole backup method. It is part of it. The idea is to backup a secondary mirror to an tape or disk based backup system on our file server using EMC Retrospect. This would provide recovery from deleted or modified files.

The need to unmount a mirror from backup comes from my desire to have perfect snaphots-in-time for backup. Backing up a live filesystem can lead to inconsistent states in the backup. For example, backing up 50G over a network can take several hours. If, for example, you are writing an article that contists of several files, and you had a draft of file #1 that was copied to the backup. A little later, while editing file #2, you decided to move some text from it to file #1. After moving this, file #2 is backed up, with the moved text removed from it. The end-result in the backup is that the text is in neither file #1 or file #2.

Backing up live file-systems has this inherent danger. One way of avoiding it is to create an instant snapshot of the entire file system to a second drive and backup from that drive. The mirrored secondary drive in the RAID system would have this snapshot, continually updated. It would have to be disconneted from the RAID set before the backup from it is started in order to preserve the snapshot's consistency.

Reply

Answer 11

Sep 10, 2006 6:08 AM in response to Kumaradas

Hi Kumaradas;

Come on now you can't have it both ways! If you are doing RAID 1 or mirroring, you don't get stripes. They come with RAID 0.

Every time I have set up RAID arrays for servers, it has always been if the disks don't match you get the slower and the smaller.

Allan

Reply

Answer 12

Kumaradas Author

Level 1

0 points

Sep 10, 2006 6:48 PM in response to Allan Eckert

See the second paragraph in the RAID 1 section at http://en.wikipedia.org/wiki/Redundantarray_of_independent_disks#RAID1 or the last paragraph at http://www.softraid.com/understandmanual.html

Mac Pro Mac OS X (10.4.7)

Reply