Windows Forum

Ask something
Discussion Locked

OS and RAID 5

0 helpful
6 replies

by prunejuice

Closed to new repliesThis discussion is closed. It stays readable, but it takes no new replies.

I am in the happy position of having funds to buy a new DC/file server.
Is there any common wisdom about where to put the OS? Specifically, if I configure 3 drives with RAID5 (hardware), is it adequate to make a partition for the OS? Or should the OS get it's own set of HDDs?
Also, if I only get 3 disks, and one fails, will the other 2 carry on until I hot swap a new one in or should I get 4 disks for no down time?

6 replies

by timwalsh

The degree of redundancy/fault tolerance you implement should be based on risk analysis of your critical data/business processes.

Couple of things to understand first:
1. Hot-swap capability is NOT implicit with a hardware RAID implementation.2. Hot-swap implies being able to remove a drive from the array without shutting down the system and without affecting the rest of the array or the operation of the OS. This requires a specialized drive backplane (or a specialized drive enclosure)that supports hot-swap. Everything (OS, RAID controller, drives, drive enclosures/backplane) must support hot-swap for it to work.
3. Raid 5 with the minimum number of disks (3) allows you to rebuild the data set from the 2 remaining drives if one goes down. You cannot continue to operate (at least not for long) on just 2 drives.

I?ll answer the easy question first: Where you put the OS totally depends on the amount of money available for the solution. It also depends on the amount of fault-tolerance required.

Your best bet (since you are buying a new server anyway) if you want/need hot-swap capability is to buy a server (from a recognized server vendor - Dell, Compaq, IBM) configured to support hot-swap. The vendor will include either a hot-swap backplane built into the server or a specialized drive enclosure with a hot-swap backplane. NOTE: This capability does cost money and is a capability you may not really need. Hot-swap is really only necessary if you have a server that MUST remain operational 24/7. If you don't have this requirement you would be spending big money for a convenience.
(continued)

by timwalsh

Assuming you don't have a 24/7 requirement, here is what I would suggest:
Priorities for spending money:
1. Sufficient storage capacity based on a minimum of a 3-drive RAID 5 array to handle twice your current storage requirements and using a single channel RAID controller.
2. Backup solution that will handle a complete backup to a single tape (or smaller tape capacity combined with an auto-loading tape library).
3. CPU/RAM: Depending on number of clients accessing the DC/file server - something mid-range between minimum needed for OS and latest/ greatest/fastest for the CPU and 512MB RAM will probably be sufficient. Probably don't need multiple CPUs unless you will also be running something like Exchange or SQL.

If you still have money left over after meeting these priorities, consider:
1. More than 3 drives for the RAID 5 array. This will allow you to continue to operate safely should one drive crash. The only downtime would be the time necessary to remove the bad drive and reinstall the new drive. (NOTE: depending on total amount of data stored, regeneration of the array (this process takes place automatically when replacing a faulty drive) could take significant time.)
2. Multi-channel RAID controllers: Create a RAID 5 array on 1 channel and use it for your data. On the second channel, create a 2-drive Raid 1 array (mirror) for your operating system and critical applications. This solution gives you the added benefit of allowing the server to stillperform its DC functions while the RAID 5 array regenerates itself (in the event of failure). If one the mirrored drives fails, the other drive can take up the slack until you replace the bad one and recreate the mirror (usually not a time consuming process).
3. If you still have money available, using a controller with more than 2 channels, you could create a mirrored RAID 5 array (2 RAID 5 arrays set up as a mirror).

by greybeard770

Software RAID is bad.

For the domain controller a (hardware) RAID 1 system is nice. If a drive fails, you can replace it as soon as a replacement arrives. Having nothing but domain controlling services on a domain controller is the way to go.

For the file server, a (hardware) RAID 5 disk sysstem may be better if you are reading large files. RAID 5 write a block of data on a disk, another block of data on the next disk, then calculates parity for those blocks and write the parity information on the next disk. That is why you need to let the RAID card processor do that instead of the system processor.

But I a fear you are talking about one unit to do both. If you lose a disk in a RAID 5 array, the system will continue to run but the performance will turn to mud. If you can allocate a 4th disk as a hot spare (not to be confused with hot swap) the system can rebuild the failed drive on the hot spare. But that rebuilding takes time and degrades performance, but slow is better than total failure.

A nice compromise might be a RAID 1 array (2 disks) for the OS and a RAID 5 array (3 disks) for the file server data. Adding a 6th disk as a global hot spare would be real nice.

All it takes is money.

by prunejuice

Sounds like general concurrence on keeping the OS separate. Thanks. I'll probably go with the 2+3 option, as I can't afford >1 machine, but can afford to spend some $$ on the one. Old server (also 2+3) will most likely come back as a DC after upgrade to W2k.

This discussion is closed to new replies.