On Tue, Aug 4, 2009 at 2:17 AM, Brian Modra <epailty@xxxxxxxxxxxxxx> wrote:
Initially, I would agree with you that placing the OS and database on the same RAID config sounds logical but once you go through some "disasters" you'll realize it's a putting all your eggs in the same basket kind of thing.
With the OS, and presumably backup software, on it's own RAID config you can recover the database, assuming you lost it due to hardware failure, without having to recover the OS. This is a nice thing especially if you're remote from your servers, like I am, and do not have the luxury of being able to pop a CD in the server's drive to load the OS again. That's just one case and not a database-admin one but I'm sure there are others.
2009/8/3 Scott Marlowe <scott.marlowe@xxxxxxxxx>
>It was an error. I wanted mirroring. But... on second thoughts, is
> On Mon, Aug 3, 2009 at 10:15 AM, Brian Modra<brian@xxxxxxxxxxxxx> wrote:
>
> Is there a valid reason you're NOT considering RAID-1 here? I hope
> RAID-0 is a typo.
there really a good reason for using a second set of disks for the OS?
Once the database is running, its surely not going to be using the OS
disk much, so why not just make a big RAID 10 array and use that for
both OS and DB... partition it as usual I mean - boot, root. Should I
use another disk for swap... for that matter, do I need swap at all...
RAM with be at least 16GB?
Initially, I would agree with you that placing the OS and database on the same RAID config sounds logical but once you go through some "disasters" you'll realize it's a putting all your eggs in the same basket kind of thing.
With the OS, and presumably backup software, on it's own RAID config you can recover the database, assuming you lost it due to hardware failure, without having to recover the OS. This is a nice thing especially if you're remote from your servers, like I am, and do not have the luxury of being able to pop a CD in the server's drive to load the OS again. That's just one case and not a database-admin one but I'm sure there are others.
>
> Then I question the expertise of your experts. RAID5 is not fine.
> It's slow, more prone to loss due to drive loss, and generally not a
> good choice for databases.
>
> I would gladly have more SATA drives in a RAID-10 than fewer SAS
> drives in a RAID-5.
>
> if someone is worried about "wasting" disk space tell them to worry
> about something else, like losing data.
On the performance argument, I wholeheartedly agree that RAID-5 is not where it's at. Sequential I/O is on-par with other RAID types but when it comes to random I/O it's one of, if not, the worst of the bunch.
From a recoverability angle, losing a disk in a RAID-5 isn't the end of the world but your world will spin much, much slower than it did while it's recalculating all those parity blocks and while doing so you're at disk of data loss if a second drive goes.
There are units out there that allow for mirrored RAID-5, RAID-5+1, to protect from multiple disk failures however at that point RAID-10 is the route to go.
There are units that 'format' the RAID group only where the disk has been allocated. In other words, if you have a 4 disk RAID-6 and 25% of it has been allocated to LUN(s) then the controller will have the parity calculated for only that 25% in use. Makes recovery quicker in an underallocated situations but there is still a window with a RAID-5 recovery where a second disk failure kills the whole operation. RAID-6 however is better in this case b/c it takes a third disk failure before data loss but you had better have a second spare waiting in the wings.
I don't believe RAID-10's are perfect either. If your RAID-10 is really 2 RAID-0's mirrored, i.e. RAID-0+1, and you have 2 disks failure, one in each RAID-0, then that's a go-to-tape situation. If your RAID-10 is really multiple mirrors striped, i.e. a true RAID-10 or RAID-1+0, you're just as susceptible to data loss except you must lose both sides of a single mirror. Not as likely but still possible.
Recovery in either RAID-10 setups is simpler than the parity RAID's in that a disk must be copied only and no parity calculated. This is still a window where a second disk failure could result in data loss.
I believe that regardless of your selection you must, must, must look at things as a 3 year solution, 5 years at the most. As those disks spin and age the likelihood of multiple failures increase. You may not have to replace a single disk in the first 3 years but should you lose power and those drives spin down and cool the odds of one or more not spinning back up are pretty good. Trust me, I've experienced that many times. Plan to replace aging units.
In the end, people will do what people will do and most likely the largest factor won't be performance, protection or recoverability but instead it will be money. If you're lucky, money isn't an issue.
Greg
