A few know that in an implementation of RAID 1 is required to read data off the second disk as well. Otherwise one or even several bad sectors may appear on the second disk of the mirror.
In this situation your RAID 1 doesn't have redundancy any more and you do not know it, contrary to the principle of monitoring the array status. There may be a similar problem with hot spare drives.
One of the causes of this problem is the array being idle most of the time. To make sure the array is still redundant you can read sectors from both disks and compare it. If your RAID operates principally under heavy load you do not encounter such a issue.
In Windows, at least in server versions up to and including Windows 2003, software RAID 1 in most cases reads from the first hard drive, rendering Windows RAID1 arrays unreliable
Showing posts with label raid. Show all posts
Showing posts with label raid. Show all posts
Tuesday, April 5, 2011
Thursday, March 17, 2011
Hot spare is a handy option, but...
It is very good to have a hot spare drive, but the hard drive used as a hotspare ought to be tested every now and then.
Suppose you build your own RAID array and assign N hard drives (including the hot spare disk) to the array. Surprisingly there exists 1/N probability that exactly the disk which is designated to be a hotspare will be the first failed disk. And now you no longer have a hot spare.
If one of the array member disks fails you find out that the hot spare disk has already failed. To avoid this you need to scan a hot spare on a regular basis or stick to RAID6E or RAID 5E/EE layout.
Suppose you build your own RAID array and assign N hard drives (including the hot spare disk) to the array. Surprisingly there exists 1/N probability that exactly the disk which is designated to be a hotspare will be the first failed disk. And now you no longer have a hot spare.
If one of the array member disks fails you find out that the hot spare disk has already failed. To avoid this you need to scan a hot spare on a regular basis or stick to RAID6E or RAID 5E/EE layout.
Thursday, February 10, 2011
DIY RAID tips
When going to build a new RAID, you need three factors right.
1. Redundancy.
RAID does not replace a proper backup. You still have to back up your data somewhere in a safe place. RAID can not save you from operator errors or fire, earthquake, or whatever is common in your area. Anyway, most RAID types would still be operational when one of the array's drives fails. These include RAID1, RAID 1+0, RAID 4, RAID5, RAID 6, and exotics such as RAID 5E, RAID 5 EE, and Diagonal Parity RAID (RAID-DP).
2. User-accessible Capacity.
The capacity of the RAID array determined by the maximum drive capacity available, number of ports on the RAID controller, and the capacity required to maintain redundancy. Should you need simple capacity calculations done for you, use this free RAID Calculator.
3. Speed requirements.
Among the redundant array levels, RAID1 and RAID10 are the perferred choice on random writes.
If the array is almost always used read-only (e.g. a media library), or a write-once-read-never pattern (like a backup), then RAID5, RAID6, and variations of those are OK. If fast random writes are required, opt for RAID 1+0.
Bonus: A little different perspective of the RAID planning and requirements.
1. Redundancy.
RAID does not replace a proper backup. You still have to back up your data somewhere in a safe place. RAID can not save you from operator errors or fire, earthquake, or whatever is common in your area. Anyway, most RAID types would still be operational when one of the array's drives fails. These include RAID1, RAID 1+0, RAID 4, RAID5, RAID 6, and exotics such as RAID 5E, RAID 5 EE, and Diagonal Parity RAID (RAID-DP).
2. User-accessible Capacity.
The capacity of the RAID array determined by the maximum drive capacity available, number of ports on the RAID controller, and the capacity required to maintain redundancy. Should you need simple capacity calculations done for you, use this free RAID Calculator.
3. Speed requirements.
Among the redundant array levels, RAID1 and RAID10 are the perferred choice on random writes.
If the array is almost always used read-only (e.g. a media library), or a write-once-read-never pattern (like a backup), then RAID5, RAID6, and variations of those are OK. If fast random writes are required, opt for RAID 1+0.
Bonus: A little different perspective of the RAID planning and requirements.
Thursday, January 6, 2011
Make a software RAID bootable?
Can I put an OS installation onto a software RAID?
Unfortunately, no. One cannot boot off a software RAID0, RAID5, or span. A hardware controller is needed to do this.
You cannot start an OS off the software RAID for the array is not properly readable before the operating system is fully started up, and the operating system itself is on the array.
The only option is to start an OS using a software RAID 1, and even that is not that simple. To boot from a "second" disk of a software RAID1, you probably need to manually copy the master boot record first, because usually the master boot record is not copied automatically.
Unfortunately, no. One cannot boot off a software RAID0, RAID5, or span. A hardware controller is needed to do this.
You cannot start an OS off the software RAID for the array is not properly readable before the operating system is fully started up, and the operating system itself is on the array.
The only option is to start an OS using a software RAID 1, and even that is not that simple. To boot from a "second" disk of a software RAID1, you probably need to manually copy the master boot record first, because usually the master boot record is not copied automatically.
Subscribe to:
Posts (Atom)