Re: What is the point of RAID?
Douglas A. Tutty wrote:
On Sat, Nov 08, 2008 at 03:45:38AM -0600, Mark Allums wrote:
lee wrote:
On Fri, 7 Nov 2008 23:29:55 -0500
"Douglas A. Tutty" <dtutty@vianet.ca> wrote:
On Fri, Nov 07, 2008 at 10:20:51PM -0600, Mark Allums wrote:
If you do only have three drives, add it to the raid1 array.
Hm, if you do that, is there any other use for the third disk than as a
spare?
I think my name did not add appreciably to the subject, here, but, to
answer the question, I didn't understand what Douglas meant. In a RAID
0, (striping) you can stripe across any number of drives. In RAID 1
(mirroring) you can mirror one drive any number of times. I am not sure
what the point would be of the latter.
Douglas, could you clarify?
With a two-drive raid1, if one drive fails, you pull it out and put in a
new drive. The array is then sync'd to the new disk. That can put a
load on the older disk (which may be the same age, brand, batch even, of
the one that failed), which could fail before the sync is completed.
With a three-drive raid1, if one drive fails, you still have a two-drive
raid1 array to protect the data while a new third drive is sync'd.
I suggested this use of a third drive (if you had it lying around)
because of the slowness of raid5.
[with snips]
At this late date, let me add
that three drives is 50% more likely to fail than two. More than fifty
percent, if I remember my statistics at all correctly. If you have a
RAID 50 running on 20 SAS drives and 4 hot spares, you better buy quite
a few for cold spares, you are going to lose a drive every two months.
At least.
The other thing is, the three drive RAID drive is slow with NTFS. I
can't speak for Reiser3/4, ext3/4, HFS/HFS+, JFS, weird layers over FAT
exotic file systems, etc.
(I would guess, still slow, since software RAID5 is done per partition
or drive, and the file system is laid over the partition.
The reason I suspect there might be differences is because that NTFS
writes metadata all over the place. On a logical drive, to be sure.
It's probably relying on LBA on the hard disk, though. I really dislike
NTFS.
On the other hand, the instant ZFS is stable in Debian, I am switching
to it. No RAID required!
MArk Allums
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