SHR on two identical disks

The set of questions from here, while SHR-specific, applies to NETGEARs X-RAID as well.

I have a DS213j set up as Synology Hybrid RAID (SHR) with 2 2TB disks. What happens if the DS213j fails? Are the disks just mirrors so that I can connect either to a PC and see it as an external drive and recover the data? If not would it be better to change to RAID 1? What are the advantages of SHR over RAID 1?

SHR means Synology Hybrid RAID. The advantage of SHR is that SHR can better utilize capacity of mixed size disks, and SHR supports some weird disk upgrades which are normally done by deleting and rebuilding the traditional array.

However, if we are talking two identical disks and no plans to upgrade, SHR is in effect the same as RAID1, so there is really no choosing one or the other.

ReadyNAS RAID Levels

ReadyNAS comes in a wide variety of RAID levels, none of them seemingly matching the standard. There are (in different ReadyNAS devices)

  • Flex-RAID
  • X-RAID
  • X-RAID2

Flex-RAID is the most simple of them all. It is a stand-in for manual configuration. Once you choose Flex-RAID, the system asks you to choose between any of the standard RAID levels, and if you go for multiple arrays, then how many disks are allocated to each of the arrays.

X-RAID and X-RAID2 both hide RAID settings from user. Both will automatically expand array if more disks are added, or if enough disks are replaced with larger ones and the array can be expanded maintaining redundancy. Internally,

  1. on one disk, it is just a simple partition,
  2. as the second disk is added, that simple partition is converted to a RAID1,
  3. at the addition of the third disk,  RAID1 is converted to a RAID5,
  4. as more disks are added, RAID5 is reshaped to accommodate additional disks.

The exact difference between X-RAID and X-RAID2 is sort of moot. For all intents and purposes, once the RAID crashed, they are the same.

Home NAS Recovery works with any of these configurations except

  1. where disks of different sizes are involved
  2. where multiple RAIDs are involved; Home NAS Recovery requires RAIDs fed to it one by one. If you have multiple RAIDs and you do not remember which disks form which RAID, your case is likely to end up in a data recovery lab anyway.

How long to rebuild?

There is a story of routine drive replacement going belly up

[I] have a Netgear ReadyNAS NV+ (RND4410)… [containing] four 1TB-disks…I changed all disks to 2Tb-disks. As it’s a X-Raid NAS I just pulled the disk from slot 1 and mounted the first new 2tb disk…[NAS] started the sync… 3 days later it still said syncing and I could still not reach the web-interface.

I pulled the plug and rebooted the NAS.

It did a check on the NAS volume and then it said booting with the LED on slot 1 still blinking.

[30 hours later it was] stuck on Booting and can not reach it in the web-interface.

despite the NAS owner specifically states that

No smart errors detected before start.

The case still looks very much like a failure of the second disk during rebuild. The most likely reason of a NAS not being able to complete the rebuild while blinking its disk LEDs is a read failure on one of the remaining drive. The X-RAID mode in ReadyNAS is essentially a RAID5, so once the disk fails, the NAS is supposed to remain online and rebuild the content of the failed disk once said disk is replaced. Observed result is that the NAS does not even remain online. Leaving aside the possibility of the new disk being a dud, another likely reason is that the second disk failed to read a block and the NAS is now locked up, endlessly retrying the read.

What could be done in this situation?

  1. Pull the new disk out and try to start up the NAS. If the NAS starts, shut it down again and try with another blank new disk. This rules out the possibility of using a dud as replacement.
  2. If that does not work, try and put the original disk back. This further rules out any  incompatibility of the new disks with the NAS. The entire batch of new disks may be DOA or incompatible. These things happen, albeit rarely.
  3. If putting the original disk back does not help, yet another attempt should be made without the disk at all (with both original and replacement out). If the NAS comes online, first thing should be made is to back up its content, most starting with the most important.

If none of this works, we have Home NAS Recovery for you. You should give it all the disks which were in the NAS all the time. That is, you’d better leave out the original disk which was removed and any replacement disks. The only set of disks guaranteed to be in sync with each other are the disks which were never removed from the NAS.