> > Slower, though ... is a bit of a strange claim. Not because it is false, > but > because the answer is complex: you can, for example, double read speed and > halve write speed, using a two disk RAID 1 array ... in the ideal case. >
I must say I'm curious about this, because I have always assumed that for a RAID 1 the write speed would be roughly the same as a single disk, not halved.. my reasoning being that both writes would occur in parallel, as with the reads.. the difference of course is that the 2 reads in parallel each transfer half the data, but the 2 writes transfers all the data each sure, you may have a little bit of overhead - issuing 2 IO instructions instead of 1, or in the case of a setup where both disks share the same bus (which is not the ideal setup) there would be contention on this bus, but halved? Is it really the case? If this is true, I guess the reason would be that the same data travels over the same bus twice before the operation can be said to be completed, therefore halving your write speed. But then this holds true for the read as well, so that despite issuing an instruction to 2 different disks, each with half the data requested, then you will meet the same contention and the data will get to you with the same speed as 1 disk.. so, if this is right, then RAID 1 compared to a single disk would be something like 1. 2 disks on 2 buses = (approx) half read time, same write time 2. 2 disks on 1 bus = (approx) same read time, double write time I honestly don't know if this is the case or not, I've certaintly never measured it and it may be implementation specific, but if not I'd really like to be shown where this is wrong.. -- SLUG - Sydney Linux User's Group Mailing List - http://slug.org.au/ Subscription info and FAQs: http://slug.org.au/faq/mailinglists.html
