Re: Newbie storage questions... (RAID5, SANs, SCSI)

On Thu, 27 Nov 2003 16:20:23 GMT, David Sworder wrote:
>     This is really great information. I apologize for the basic questions,
> but I've only been examining this stuff for the better part of one day. :)
> Let me ask you a few follow ups...
>
>> Just to put a number on things, let's say you've got a RAID 5 array of
>> five drives with 5ms typical access time (pretty optimistic).  So each
>> drive can do about 200 I/Os per second.  So you could sustain
>> something like 1000 random reads per second (with zero writes)....
>
>     I don't quite understand this concept. You've got five drives, each of
> which can handle 200 I/Os per second. You're multiplying 5*200 to get 1000
> IOPs for the array. I understand your calculation but I'm not sure why it
> works as you state. In a trivial example, let's say the RAID controller is
> instrutcted to read 5 bytes of data. This is considered one IO by the RAID
> controller, but doesn't the RAID controller then have to issue *5* read
> commands, one to each disk? My understanding of RAID (as it applies to
> reading data) is that the 5 disks would always be accessed simultaneously in
> order to speed up the read process. So for each IO read-request that the
> RAID controller receives, it has to issue 5 IO requests, one to each drive.
> So it seems that the RAID controller would *still* be limited to 200 IOPs,
> regardless of how many drives on are on the array. Why is it that you say
> the reality of the situation is that the RAID controller can actually handle
> 1000 IOPs? I don't understand.
>
Here comes the term "stripe size".
This is the number of consequtive bytes allocated on the same disc.
Depending on your performance requirement you will chose a small or large
stripe size. (8k-64k or even much larger)

/hjj