Hi
Look! I'm responding to the original question, not getting involved in
an argument. Woo!
On Thu, 2008-03-20 at 19:35 +0000, Tom Brown wrote:
> I wonder what people think for a RAID level for their mail stores?
It depends entirely on circumstance. That circumstance comprises
hardware, number of drives, OS, network load, peak required throughput,
what apps are sharing the array, required space, future growth, whether
the system is read- or write-heavy, the size of the files, and a
multitude of other factors (many of which Bill Hacker has already
mentioned in the spiralling "which is best" thread).
> I can only have RAID 1 or RAID 5 and i dont have many users ~500 so what
> do people think? This will be on a hardware RAID card but the RAID
> channel will not be dedicated to the mail store ie there will be another
> RAID disk sharing the channel.
You may want to refer to some of the RAID resources on the web. Googling
for "raid levels" is a good start.
In essence, given that you have a choice of RAID1 or RAID5 you're
choosing between the following (or not; depending on the number of
spindles you require):
RAID1
Mirroring.
A given pair of disks act as a single drive.
Writes can be slower than a single disk (all drives in the mirror have
to report success for a write to complete).
Reads can be faster than a single disk (reads can come from any disk in
the mirror; where files exceed a single block size the file can be read
from multiple disks simultaneously).
No loss of data with failure of a single disk.
RAID5
Stripe + Parity
The data and parity blocks are striped across all spindles within the
array.
Poor write performance - each write takes multiple operations; read/read
parity/calc parity/write data+parity. A good controller does this in RAM
(cache) and flushes to disk some time later. A better controller has
battery backup so in the event of a power failure it can flush the cache
after power is restored, leaving the data intact.
No loss of data with failure of a single disk.
> Just want to lay the spindles out 'correctly'
How many are you intending to have?
In my experience, 500 users is fairly small and will not hit you so
hard; RAID5 will probably be a good compromise for you since it will
give you the maximum amount of data for a given set of spindles (above
3). RAID1 essentially gives you half of your theoretical capacity; RAID5
is harder to calculate but, for example, using a 6-spindle array of
100GB disks with no spare drive you'd have in the region of 475GB
available (this will shrink or grow according to vendor parity
calculations, "right-sizing", and a number of other factors).
It's also worth remembering that vendors sell disks according to "Giga"
equalling 1000, not 1024 - so your actual disk size is likely to be
slightly smaller anyway. As an example, a RAID1 mirror of two 750GB
vendor-sized disks shows me:
Array Size : 730860992 (697.00 GiB 748.40 GB)
The GiB being the actual Gigabytes in base 16 rather than base 10.
Anyway, that's a digression.
As you mention the fact that you only have level 1 or 5 available, how
many spindles can your hardware controller have attached?
Graeme
