It is clear that disk striping can improve data transfer rate when the strip siz
ID: 3636379 • Letter: I
Question
It is clear that disk striping can improve data transfer rate when the strip size is small compared to the I/O request size. It should also be clear that RAID 0 provides improved performance relative to a single large disk, because multiple I/O requests can be handled in parallel. However, in this latter case, is disk striping necessary? That is, does disk striping improve I/O request rate performance compared to a comparable disk array without striping?b. Consider a 5-drive, 500GB-per-drive RAID array.What is the available data storage capacity for each of the RAID levels, 0, 1, 3, 4, 5, and 6?
Explanation / Answer
a) Consider a disk array that does not use striping. If multiple I/O requests are issued for files on different disks, then performance is the same as a striped disk array. However, the advantage of striping is that it improves the chances that multiple I/O requests will in fact be for data on different disks. Without striping, all the files from a given directory will be on the same disk, so a single user operating in a single directory will be using one disk instead of all the disks in the array. Likewise, it is likely that all user files will be on one disk, and all binaries on another, so that multiple users accessing their files will use the same disk. Disk striping is a simple, automated way for the administrator of the array to ensure that files are spread out across all the disks.
b)
Onboard vs Dedicated aside (understandably, Raid 5 requires more processing power for parity), I had a question/statement in regards to all the Raid recommendations I have seen here. Why is Raid either not recommended off the bat, or the recommendations that do come out for Raid 1 or Raid 10?
The way I see it is as follows. For a theoretical array of 4 500GB drives, the following statements should be true as I understand them.
Capacity
Raid 0 = 2TB (capacity = total of all drives, minimum 2)
Raid 1 = 2x 500GB (2:1 ratio, minimum 2 per set)
Raid 10 = 1TB (2:1 ratio, minimum 4)
Raid 5 = 1.5TB (capacity = total of all drives minus 1, minimum 3)
Redundancy (NOT BACK UP)
Raid 0 = NONE
Raid 1 = Single Drive Failure per mirrored set
Raid 10 = Single Drive Failure per mirrored set
Raid 5 = Single Drive Failure
Performance
Raid 0 = Fastest, performance increases with drive count
Raid 1 = Possible boost in speed for READ, no gain to possible loss (if noticeable) elsewhere
Raid 10 = Similar performance to its equivalent drive Raid 0 (ie 3 Drive Raid0=6 drive Raid 10)
Raid 5 = Between Raid 0 and single drive performance, goes up with drive count. processor intensive
With these statements, it seems to me that on a price vs capacity+performance comparison, Raid 5 comes out as the winner, the only fault being the extra processing power required (on software raid at least) for parity information on writes. otherwise, for the price of a single extra drive, one can have a similar level of fault tolerance, 66% of the available capacity (this also increases as drive count goes up, 75% available in a 4 drive array etc.) and descent read/write performance as compared to a 2 drive Raid 1 array with only 50% available capacity. This also allows gamers to have their boost in performance and have redundancy to boot. With more and more software based controllers offering this as an option, why not?
Disclaimer for those reading this to gain an idea of what array to go with, Raid for redundancy does NOT cover loss of data due to formatting, accidental deletion, viruses or any myriad of events leading to data loss. these do however offer protection against data faults and hardware failure (except Raid 0 with ZERO protection). having a Raid array is NO EXCUSE to not back up important data!
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