Time to Rebuild vs Backup?

[bfactor]

After reading through the reports on NAS solutions I am left with the impression that it could take significantly longer to rebuild a disk in two-disk (e.g. QNAP TS-239 Pro) NAS than to back up to eSATA external Disk. My questions are not particular to QNAP (but are particular to 2-disk NAS solutions).

The numbers for some of the new models seem to be in the 40+MB/s range for backing up to eSATA. The best indication I found on smallnetbuilder of replacing 1 disk in a RAID 1 array was 30GB in 2 hours (or 4.2MB/s --- one tenth the speed). This article was not for a 2-disk solution but I couldn't find any references for rebuild speeds in a recent 2-disk configuration.
http://www.smallnetbuilder.com/content/view/30324/75/1/3/

That seems really slow to me. Can it be done closer to the eSATA backup speed if I accomplished it differently (setting to read-only or taking offline, etc)?

What is your experience on rebuilding 500GB in a RAID-1 2-disk array?

I wanted to ask the above questions before offering my general goal below.

I am newly supporting several small businesses and non-profits that have a range of 500GB - 900GB of storage. I was thinking of setting up a 2disk RAID-1 array at each of their sites.

I plan on visiting them every other week and wanted to replace (alternating which drive) one of their 1TB drives with an extra clean one (so each drive would be replaced 13 times in a year).

My thoughts were that if they had a NAS failure (both drives lost) I could just come back with another duplicate NAS, put the last drive I took in it with another clean drive, and they would be back to running (although at most 2 weeks behind with their data).

So this brought me to my earlier question. If a rebuild is 4.2MB/s it would take roughly 100000 seconds (or 27hours) to rebuild 420GB. I am guessing my numbers must be very wrong.


Thank you for your patience in reading through my post,

 

[thiggins]

Manufacturers must decide how to set process priority between serving files and rebuild. Each manf trades this off a different way. Some allow you to set the rebuild priority (I don't recall which offhand, sorry.), but most don't.

I usually note rebuild times during my testing, but don't remember to include them in the reviews. Rebuild times for four 1 TB drives in RAID 5 in current-generation NASes typically run 4-8 hours. The QNAP TS-219P that I recently tested took 3.5 hours to rebuild a RAID 1 array of 1 TB drives.

The most important determinants of rebuild time are volume size and disk activity. If the drive is used during rebuild, it significantly increases rebuild time. That 3.5 hours could easily double, if not triple if users are reading and writing to the volume that is being resync'd.