Data Recovery Weekly

There was an incident recently, which involved a near-loss of some important git repository. The incident involved the distributed system of multiple servers, one being the designated master and multiple slaves. Slaves pulled data automatically, and that was apparently done quite often. You guess what happens next, the master copy got corrupt, and before anyone knows all the slaves pull the corrupt copy. In the follow up ( here ) they state that they have a backup system in place which is principially different from a RAID 1. That is obviously not so. Any system with automatic replication is subject to the following failure mode - the master copy is damaged, and the damage is then automatically replicated to slaves (mirrors). The automatic replication systems are designed around the assumption that all the master failures are fail-stop - the master either fails mechanically and ceases to perform completely, or the master can detect any and all cases of corruptio...

The computer with Asus motherboard based on ICH8R has worked flawlessly for about 1.5 years. Two 250 GB hard disks were in RAID1 divided into two partitions. Two weeks ago I heard about a problem (the request to press a key when loading) which don't lead to subsequent problems in the operation. When the system is starting up, the message "Primary HDD not found. Press F1 to resume" is displayed. Pressing F1 leads to usual system start with the message "New device was found ...". When I started to sort out what happened I found out that RAID controller switched over to IDE mode. Disk Management displayed copies of logical disks (originally there were C:, D:, -- but now E: and F: are added with the same sizes and labels). The freshest file dates on the copies coincided with the date when start up problems arose. When I switched the controller to RAID, RAID1 appeared immediately with the name specified by me in RAID settings. The state of RAID 1 was shown as "N...

Surprising, isn't it? Actually, RAID does indeed increase failure rate. If you take MTBF, MTBF decreases with more disks. Even if RAID5, mean time between disk failures decreases. In a fault-tolerant storage, time between failures (MTBF) does not matter. What matters is time between data loss events. This is called either mean time to data loss (MTTDL) or mean time between data losses (MTBDL). You know you can setup a three-way RAID1 (three mirrored copies instead of two), i.e. the mirror can have more than two disks. So, let's imagine a RAID1 of infinite number of disks. This unit will have an MTBF of zero, because at any given moment one of the infinite number of disks is failing. It will also be continuously rebuilding while still delivering infinite linear read speed. Still, this imaginary device will have zero probability of losing data because of the disk failure, because the infinite number of disks cannot all fail at the same time.

Is it possible to move a RAID 1 (mirror) set between controllers? In RAID1, two disks are exact duplicates of each other as far as user data is concerned. The RAID controller metadata is probably disk-specific, but that does not practically matter. Because the disks are identical, there is no point in moving entire RAID 1. Move one disk, then make a mirror from it. The question should thus be restated as Is it possible to use one member disk of RAID 1 with a different controller? The answer depends on a controller in use. With most controllers, you can connect the RAID 1 member disk in whatever way you like and the data would be accessible. There are however some controllers which append their metadata before the user data on the member disks. With these, the direct migration is not possible. Use either a same model or a compatible controller, or use something like TestDisk or DiskPatch to do a partition recovery and make the filesystem mountable again....

The common wisdom says that RAID 5 requires a minimum of three disks. If one thinks of it, it is not true. You can create a RAID 5 of two disks. With even parity, the array becomes a mirror (RAID 1), and with odd parity (although nobody ever uses that), becomes a RAID 4 of two disks. And yes, the same applies to a RAID 4 of two disks.

Actually, RAID 1 has the same write hole  problem as RAID 5 does. Should the power fail after one disk is updated, but the other is not yet updated, and then the first disk fails, there will be data corruption. As usual, scheduled synchronizations of the array reduce probability of this effect causing any practical trouble.

Can I use one 10,000 RPM and one 7200 PRM hard drives in RAID 1? Yes. In RAID1, that would not cause any noticeable issues.

Do not underestimate the software RAID. Software RAID would typically provide the same, of not better reliability than an entry-level server hardware RAID controller. Software RAID is easier to move around from server to server. It does not require an identical replacement controller in case of a controller failure, which is sometimes a hinderance with server hardware. In RAID 0, RAID 1, and RAID 10, the hardware RAID controller does not provide any computational power benefit because there is nothing to offload. Most modern SOHO and small business NAS devices, like Synology, QNAP, or NetGear, use the software RAID (Linux/mdadm).