Hi Dale,
> (1): 4x 73GB in RAID 1+0 + 2x 146GB disks standalone for backups
> (2): 4x 73GB in RAID 1+0 + 1x 73GB disk as hot standby + 1x AIT
> ...
> After reading the docs, there was a note that splitting the disks in an
> array between two channels would improve performance.
In pure sequential operation you could fully use the bus bandwidth of one channel with 5 really good 15000rpm drives.
However your data are mainly mail dirs/queue and database which typicaly generate random I/O. In this case the bus bandwidth is not performance critical, it is the time to access the data which really matters. The more disks you have, the better will be your performance in this case (the RAID 0+1 you choose is the best choice). 15000 rpm drives rather that 10000rpm will help too.
With 4 disks in random IO you won't reach the bandwidth limit so splitting this RAID on 2 channels will not improve the performance. Nevertheless it will improve the reliability of your solution in the way that if you lose a channel the second one will be sufficient (the RAID 1 in the 0+1 is automatically made over the 2 channels to assure this possibility of redondancy - the risk of failure of only one channel is really really low anyway).
The backup is going to generate more sequential operations. The backup device will be one disk or a AIT tape device.
In the case of the tape backup, the maximum speed you could expect is 24Mb (in 2:1 compression). It is really low so no problem of bandwidth.
In the case of the disk drive, in the best case you will use something like 2x 60MB/s. If it is on the same channel 200MB/s are still left. I honestly don't think that on production you will be able to reach this value with random I/O on the RAID0+1.
So no problem of bandwidth neither in the case of the backup, you can let all on the same channel.
However the backup operation will access the RAID0+1 so it could potentially slow down the production. So doing it in a dedicated time is a good idea. With a AIT100 (2) you will be able to do about 70Go/hour so something like 2 hours would be necessary to backup your data.
To resume you won't have bandwidth problem whatever the channel configuration you will choose.
Splitting you RAID0+1 in 2 channel will slightly improve the relability.
Backup during non production time is a good idea if you are looking for the best performance during production.
> [...] where files are always synced to disk, [...]
The SA6402 has a read/write cache with battery. So when a sync is done the data could still be in the cache of the array controller, waiting for the best time to write its. So the performance will be improved. A battery is connected to this cache so in case of bad shutdown of the server the data still in cache will eventually be written on the next power on of the server without data lost.
Regards,
Pascal
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