RAID1 Bottleneck

[etftw]

Hello,

I have just bought a new NAS (http://www.amazon.co.uk/Netgear-RND2110-ReadyNAS-Duo-1000GB/dp/B0016KVW96) and set up two drives in it in RAID1. I connected the NAS via a gigabit connection but the transfer rate won't seem to budge over 30mb/s when reading and tends to get stuck at around 15mb/s when writing.

The primary drive is a Seagate Barracuda which runs at 7200RPM, so I think that drive should be able to hit around double what I am getting at the moment; and the second drive is a Samsung EcoGreen F2 which according to these benchmarks should be performing quite well: http://www.tomshardware.com/reviews/1.5tb-hdd-caviar,2331-6.html

My question is - does a RAID1 array copy all the data to the secondary hard drive at the same time as it is being copied to the primary drive? Therefore giving a theoretical reduction in performance?

The main thing I am struggling to understand is the read speed more than anything, so if anyone can shed some light on that it would be appreciated.

Thanks :)

[caperjack]

likely cause is the 2 different drives ,you should for best results not mix and match drives when in a raid ,the Samsung ecogreen is slower than the seagate , the raid will work at the speed of the slowest drive

RAID 1 performance

Since all the data exists in two or more copies, each with its own hardware, the read performance can go up roughly as a linear multiple of the number of copies. That is, a RAID 1 array of two drives can be reading in two different places at the same time, though not all implementations of RAID 1 do this.[4] To maximize performance benefits of RAID 1, independent disk controllers are recommended, one for each disk. Some refer to this practice as splitting or duplexing. When reading, both disks can be accessed independently and requested sectors can be split evenly between the disks. For the usual mirror of two disks, this would, in theory, double the transfer rate when reading. The apparent access time of the array would be half that of a single drive. Unlike RAID 0, this would be for all access patterns, as all the data are present on all the disks. In reality, the need to move the drive heads to the next block (to skip blocks already read by the other drives) can effectively mitigate speed advantages for sequential access. Read performance can be further improved by adding drives to the mirror. Many older IDE RAID 1 controllers read only from one disk in the pair, so their read performance is always that of a single disk. Some older RAID 1 implementations would also read both disks simultaneously and compare the data to detect errors. The error detection and correction on modern disks makes this less useful in environments requiring normal availability. When writing, the array performs like a single disk, as all mirrors must be written with the data. Note that these performance scenarios are in the best case with optimal access patterns.

Below copy /past is from here .http://en.wikipedia.org/wiki/Standard_RAID_levels#RAID_1_performance

RAID 1 has many administrative advantages. For instance, in some environments, it is possible to "split the mirror": declare one disk as inactive, do a backup of that disk, and then "rebuild" the mirror. This is useful in situations where the file system must be constantly available. This requires that the application supports recovery from the image of data on the disk at the point of the mirror split. This procedure is less critical in the presence of the "snapshot" feature of some file systems, in which some space is reserved for changes, presenting a static point-in-time view of the file system. Alternatively, a new disk can be substituted so that the inactive disk can be kept in much the same way as traditional backup. To keep redundancy during the backup process, some controllers support adding a third disk to an active pair. After a rebuild to the third disk completes, it is made inactive and backed up as described above.