I plan to build some storage schema but I have to anticipate downtime, maintenance and rebuild Time.
Some guys said to me that a 10 disk RAID6 of 10To (SATA) rebuild will last for about a week ! and some company policy ask for stopping activity on Array when rebuilding.
If I use RAID 5 or 6 or 5+1 or 6+1 is there an approximate formula that can give me hint on rebuild time depending on disk size and type (SAS/SATA/SSD).
something like rpm x size(Mb) x type-factor x nb-of-disk …
I would like to be able to anticipate all rebuild time scenario depending on Size/type of RAID/type of Disk.
I know it may depend on hardware quality, but let says I am out of dedicated hardware like 3PAR / STOREWIZE / NETAPP or likes. I am using conventional servers with traditional SAS or SATA drives with software RAID.
Best Answer
You can calculate the best-case rebuild rate fairly simply: as rebuild is sequential, the needed time is
capacity / transfer rate. For example, rebuilding a 10 TB disk with a 200 MB/s transfer rate needs at least10000000 / 200 = 50000s = ~14h.Now take this result and trow it away, as it is an overly optimistic scenario: it suppose 100% disk availability for the rebuild operation and totally sequential reads/writes. Toss in the mix some non-rebuild (ie: application) load, cap the rebuild itself to 30% (to not grind other applications to an halt) and you are suddenly in the 10x (eg: a week) rebuild time.
These long rebuild times are the reason while I avoid RAID5/6 in many system, favoring mirroring instead. Anyway, with such big drives, absolutely avoid RAID5, which is too much exposed to double failure and/or URE issues.
If you want to play with the number, give a look here