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Old December 29th, 2004, 12:15 AM   #1
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To mirror or not to mirror?

Since we just pretty much lost a huge chunk of media due to a bad drive, we've been thinking of setting up a SATA array of 4 drives as either - two sets of stripped drives - or two drives that mirror each other. Frankly, I'm not sure of the advantages of mirroring. I prefer striping for speed of media retrieval, with an external FW drive for daily backups of consolidated projects. (We use Avid Xpress Pro.)

Ive been using stripped drives for years but I have little experience with mirroring drives and I could use some input on the pros and cons of either method stripping for speed; or mirroring for constant backups.

Thank you,
Oz
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Old December 29th, 2004, 09:03 AM   #2
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Hello again Ozzie!

Important: mirroring IS NOT the same as backing up!!!

Let me explain. With mirroring you are using however many drives
as needed to mirror an active set of drives to another set. You
CAN also do this TOGETHER with striping! The choice to mirror
(usually) is made for the following:

1. you want to protect against harddisk failure

2. the system must have minimal (preferably none) downtime (which you can do with the right mirroring solution!)

However, as said this is NOT a backup for the following critical reason:

Your data is not protected against corruption

What I mean by this is that if any of the following happens your
data might be corrupted:

1. a user deletes a file (it will be deleted of the mirror as well)

2. a virus deletes or infects files (same as #1)

3. a poweroutage corrupts things (not very probable, but if this where to happen your RAID set might get destroyed)

4. a (internal/external) fire may take your system down and all of the drives

5. a thief can steal it

Etc., you get the idea. This is why backups are so important and
depending on importance they can even be moved off-site (take
a firewire drive/tape with you, back up over the internet to another
site etc. etc.) for greater security.

Ofcourse such things might be trivial and usually are in the video/
film business as long as the original tape/film is stored safely in
a fireproof place (off-site is better).

I just wanted to point out the difference to you so you understand
what each technique does!

If you have 4 SATA drives of let's say 200 GB you can do both
striping and mirroring! You create 1 set of discs as a stripe where
you get 400 GB. You then have 2 drives remaining to mirror the
striped set to get striping + mirroring.

Then a nightly (automated) backup of the project/EDL files and
any other material that is not retrievable from another place can
be stored to another computer, sent over the internet to a private
ftp server running at your home, e-mailed (if not too large) etc.

What you can do with your RAID system (striping, mirroring, a
combination of both) depends on what it supports.

If you have more questions let me know!
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Old December 29th, 2004, 10:21 AM   #3
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Rob,

Thank you for your very clear explanation. I tried to explain something like that to our editors but, since I had no idea of the whys and wherefores (I was going on half-baked hearsay), I could not make a convincing argument.

You explanation does leave room for mirroring. Our problem has not been viruses or theft. It has been data corruption or drives just going physically bad. I understand mirroring will not do anything to avoid data corruption since both drives will suffer, but it will save us in case of a drive going bad.

You can be sure your reply will be printed and posted in our edit rooms today.

Oz
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Old December 29th, 2004, 02:00 PM   #4
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Hello O,

I have set up a fileserver for NLE use. I use:

4x 250gb SATA drives
Promise FastTrak 150 SX4 RAID5 / 256mb controller

250*4gb = 1 tb - Raid5 gives me 750gb working storage

It will do a full rebuild if I loose ONE disc, if I loose 2 discs at the exact same time, then I'm f*cked :-) + The controller has extended S.M.A.R.T capabilities which will warn me when a disk is starting to get bad ( more re-reads, slower etc ) so that I can change it before it crasches.

The onboard 256mb cache gives me excellent reading speeds which are at the same level on the whole volume.

// Lazze \\
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Old December 29th, 2004, 02:21 PM   #5
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XP is RAID 5 capable if you tweak it a littlebit.
the advantage is where mirror is pure loss (the mirror disk has no use until you crash the other one),the RAID 5 gives more space and better speed.
The only drawback is you need to spend a lot of time to rebuild a lost disk from the other (up to several hours for big disk).
The good thing with the XP RAID is yo can use any kind of disks (let say 4 on SATA and 2 on IDE, even with different capacities (but the smaller will be the rule)).
Usually raid hardware solutions are very finnicky about the drives, all must be same model and same interface, usually only SCSI etc....
Here you can even put your disk into another pc, even with a different motherboard model, and it still works. If you know how RAID is tricky on server you can say this is a little miracle.

see tomshardware web site who explain and compare the RAID 5 feature of XP. it is amazing that this feature is disabled in this product.

http://www.tomshardware.com/storage/20041119/index.html
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Old December 29th, 2004, 04:07 PM   #6
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WinXP RAID???

Giroud,

I'm not sure using WinXP software RAID is a good idea for video editing systems. It may not be fast enough. Here's a quote from the conclusion section of the article on Tom's Hardware site:

Quote:
At the end of the day there is one downside left: Windows RAID 5 by far does not work as fast as hardware-based solutions. However, file servers do not always need to be as quick as possible. If you need to set up a redundant file server for occasional access and little traffic, a Windows RAID should definitely be an option to consider. But be careful: Once the RAID is rendered, you cannot transfer it to any of the hardware RAID controllers. Changing your mind this will always require reinstalling the whole array.
For my money, hardware RAID is the only way to go for both reliability and speed.
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Old December 29th, 2004, 04:24 PM   #7
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when speaking RAID on file server , the access speed and bandwith are far bigger than what you would expect for regular video editing (except if you are working on HD uncompressed).
So regarding the final comment of tomsharware, i think if you are editing DV , the throughput of the drive will be anyway better in RAID than the one you would get from a standalone drive (i.e a 200G 7200 rpm).
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Old December 29th, 2004, 05:43 PM   #8
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<<<-- The controller has extended S.M.A.R.T capabilities which will warn me when a disk is starting to get bad ( more re-reads, slower etc ) so that I can change it before it crasches. -->>>

Lars,

Can you tell me more about SMART? That souds like a most useful monitoring capability. When our drive went bad, the editor had been experiencing a myriad of problems which could have very easily have been the symptoms of a dying drive if we had only been able to diagnose it.

oz
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Old December 29th, 2004, 05:47 PM   #9
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usually SMART is the first feature you disable on drives for video, because the make them slower and noisier, but for sure it would be a nice stuff if there was not this payload.
Again for the no-brainer solution, mirroring is very efficient, but at the doublecost for the drives (but they are so cheap now).
RAID 5 is not for techies only, but require more skills and cost more because you need at least 3 drives , while the price per Meg is better than mirroring.
So if you just want your 250mb data safe, buy 2 x250 gig and mirror it. if you need more space, buy 3 x 200gig drives and you get 400 gig safe and fast.
for mirror or for RAID , always purchase the spare drive at same time, because they will soon become an hard to find item.
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Old December 30th, 2004, 07:27 AM   #10
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I wouldn't trust SMART anyway. I've had it enabled on all my drives
(that could) and it never did anything for me when a drive went
bad. Ie, no warnings whatsoever.

I forgot to mention RAID 5 indeed (basically because you where
asking about mirroring). Where mirroring needs the same amount
of drives as you are going to mirror, RAID 5 does not.

RAID 5 gets by with 3 drives since it uses parity instead of full
on mirroring. HOWEVER, you really DO NOT want to do RAID 5
in software because it is very CPU intensive. RAID 5 should be
done on a hardware card that REALLY DOES THE RAID 5 ONBOARD
(not ALL [especially cheap ones] hardware cards actually do
RAID 5 onboard if they support it), make sure that it does if you
go down that road.

But even with striping and/or mirroring it is far better to do it on
a hardware board (not that expensive these days anyway, RAID 5
hardware might be) to make sure your server/computer is not
taxed with that as well. It usually also makes it far easier to rebuild
an array (usually it can stay online, although slower as well) when
a drive goes belly up.

For the serious business you are in Ozzie and the problems you
have had I would not take any chance and go with a full hardware
setup.
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Old December 30th, 2004, 12:47 PM   #11
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Hi all,

All have their opionions :-)

For me:

1. Hardware RAID 5 is good - software I wouldn't recommend ( cpu intensive + "feels unstable". )

2. SMART is good if you have SMART enabled discs + SMART enabled controller.

3. HW RAID 5 has other ways of testing drives and will report problems through driver/gui as well.

OT:

Most drives will live a long and happy life if you provide them with:

1. Shake-free environment
2. FAN FAN FAN - they DON'T like getting hot!
3. A good and stable PSU

// Lazze \\
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