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.

  • Said company policy may make sense to some extent with RAID 5. But already with RAID 6, a rebuild of one failed disk, when the array would still work with two failures, it seems unnecessarily restrictive. The more disks you use, the more often there will be a disk failing, and you do not want the complete business come to halt for several hours. With hot-spares, a rebuild in the background may even have completed or almost completed before you even have the chance to communicate a company-wide shutdown of all relevant activities ... – Hagen von Eitzen May 19 at 10:57
  • PLEASE don't use R5, it's dangerous and no professional would use it for disks >1TB, you WILL lose data - this isn't option either, it's well documented fact. – Chopper3 May 19 at 19:06

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 least 10000000 / 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 moo much exposed to double failure and/or URE issues.

If you want to play with the number, give a look here

  • OP said he'd (have to) stop other activity on the array ... – Hagen von Eitzen May 19 at 10:49

The theoretical absolute minimum rebuild time is the time needed to write a complete disk worth of data : the capacity of a disk divided by the average sustained write speed a disk can maintain without cache.
(Note: that average sustained write speed will probably be not even near the performance numbers quoted in the specs.)

Larger disks take longer.
Slower disks take longer.
Parity calculations take extra time.

Real world numbers will vary but will certainly be (much) larger and depend on your RAID level , the number of remaining disks, load on the system while the array rebuild takes place, the controller etc.

Also see What are the different widely used RAID levels and when should I consider them?

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