Just a couple of RAID 1 questions:

a)If I set up a RAID 1 system with two hard disks, what is possible in terms
of swap partitions? It seems that I kind of need a third disk ...

Can swap partitions be RAID as well?

The reason I'm asking is that if a hard drive containing a swap partition
dies, the system is pretty well dead as well. With only two hard disks, it
isn't clear the best arrangement.

b)I've noticed that on Dell's web page, if one configures a server for RAID
1, it typically requires a separate disk controller. Why is this? Does
RAID 1 require special hardware support?

Thanks, Dave A.

David T. Ashley wrote:

> a)If I set up a RAID 1 system with two hard disks, what is possible in
> terms of swap partitions? It seems that I kind of need a third disk ...
>
> Can swap partitions be RAID as well?

Yes it can, but you don't gain much of from that as the data in the swap is
more or less temporal.

> The reason I'm asking is that if a hard drive containing a swap
> partition dies, the system is pretty well dead as well. With only two
> hard disks, it isn't clear the best arrangement.

The system can manage quite well after loosing the swap disk (if you have a
lot or ram, you most likely won't be using the swap at all), but in your case
where you have your system on the same hard drive as the swap, your system
will be gone until swap the hard drives.


> b)I've noticed that on Dell's web page, if one configures a server for
> RAID 1, it typically requires a separate disk controller. Why is this?
> Does RAID 1 require special hardware support?

No, you don't any special hardware for using RAID 1, when using Linux the only
requirements is two hard drives, as you can use softRAID (software based
raid). Most motherboards for home end users with RAID has a "low quality raid"
and most people seems to recommend the softRAID over the motherboards raid.

If you are looking at the DELLs servers, here you usually have a good hardware
based RAID, those costs more money, you attach a proper SCSI hard disk array,
but yes of course you have to spend a lot more money and may not be something
for a home user.


--

//Aho

David T. Ashley wrote:

> Just a couple of RAID 1 questions:
>
> a)If I set up a RAID 1 system with two hard disks, what is possible in
> terms
> of swap partitions? It seems that I kind of need a third disk ...
>
> Can swap partitions be RAID as well?
>
Yes, they can, but it is not recommended.

> The reason I'm asking is that if a hard drive containing a swap partition
> dies, the system is pretty well dead as well. With only two hard disks,
> it isn't clear the best arrangement.
>
Most of the times, when a harddisk dies during work, it isn't (just) the
plattern that fails - it will bring down the ide or sata controller, at
least the driver, as well, making the system almost stall or even need a
hard reboot.
After a reboot, the content of your previous swapspace is not that important
anymore ;-)
You may have two swap partitions unraided, one on each drive, and linux will
manage to load-balance their usage.

> b)I've noticed that on Dell's web page, if one configures a server for
> RAID
> 1, it typically requires a separate disk controller. Why is this? Does
> RAID 1 require special hardware support?
>
There are real hardware raid controllers - the OS will only see ones scsi
disk, so you have not much choice for the swapfile either. These devices
contain a bios and a dedicated processor. Therefore they are not really
dirt cheap, and they are not part of common mainboards either. Monitoring
is a proprietary application, if even supplied for linux.

Then, there is bios-raid pretending to do the same, but it needs a OS driver
module once control is at kernel level. These are most "mainboard raid
controllers". The workload is on the CPU. Changing the kernel implies
recompiling the module if it's not already included. Monitoring may be a
proprietary solution as well.

Third, you (and I) probably have linux software raid, which lets the CPU do
the work as well, but allows full control (you may have 3 or more disks,
the /boot partition raid1 with spare, others raid5, /tmp raid0 and so on)
and standard mdadm monitoring features.

On Sat, 19 Jul 2008 00:51:40 -0400, David T. Ashley wrote:

> b)I've noticed that on Dell's web page, if one configures a server for
> RAID 1, it typically requires a separate disk controller. Why is this?
> Does RAID 1 require special hardware support?

If you have one controller and two drives one of the drives can fail.
If you have two controllers and two drives one of the controllers or one
of the drives can fail.

It's just an added level of redundancy... and depending on the hardware,
may improve performance.