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Re: RAID 0 risky ?

Thedore Knab wrote:
> Is RAID 0 that risky anymore for data storage (IMAP mail files) ?
> I figure that under normal wear and tear a drive should last about 5 years.
> Does this sound right ?
> I have 3 IBM SCSI 18GB drives.
> With RAID 0, I get 51.5GB of storage space.
> With RAID 5, I only get 37 GB of space with 20% wasted overhead.
> RAID 0 and RAID 1 are less work for the disk volume than RAID 5.
> So in an ideal world, volumes with RAID 0 or RAID 1 will last longer than
> volumes in RAID 5.

No. In RAID 5, you can lose one drive, and still keep running. In RAID 0
(striped), the loss of one drive will kill your volume. Let's say the
odds are 1 in 10 of a drive dying in a particular year. If you have 5
drives in RAID 0, you have a 50% chance of your volume crashing (well,
that's probably not exactly right, but my statistics abilities are a
little rusty). If you have 5 drives in RAID 5, you have a 50% chance of
_one_ drive dying.

I'm not sure that there's significantly less work for RAID 0 than RAID
5. RAID 1 will definately have less reads on any particular drive than
the other two (approzimately 50% will go to one disk, 50% to the other),
but will have greater writes (100% of writes will affect both disks on
RAID 1, it's possible some writes will not affect every disk on RAID 5,
I think).

Since you're looking at IMAP mail files, the data is probably
critical... too critical to trust in a single drive failure. You're
probably most concerned with read performance (since users will notice
lag in reading email... deleting messages can plod away just fine,
writing to the mail files is done by the MDA). Take a look at how much
space you need, or are likely to need in the future. If you need a lot
of space I'd go with RAID 5. If you want to really push the read
performance, buy another drive and go RAID 1.

I'd stay away from RAID 0 unless it's fairly non-critical data, and if
you really need the throughput. An example I can think of would be
something like a web cache.

> Thus, it would be less risk to use RAID 0 or better RAID 1 than RAID 5.

Absolutely not. RAID 0 is the highest risk. RAID 0 is actually higher
risk than a single drive.


Rich Puhek               
ETN Systems Inc.         

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