Reminder -- RAID 5 is not your friend

[Daniel Feenberg]

On Mon, 15 Mar 2010, Kent Borg wrote:

> Richard Pieri wrote:
>> And neither is RAID 1.  Except when you get lucky.
>>
>> I had a failure over the weekend.  Two mirrored pairs, A1/A2 B1/B2 configuration.  A2 and B1 failed simultaneously.
>
> Sounds like it is *disks* that are not your friend. And, that they hate
> you enough that your use of raid isn't enough to save you.
>
> My conclusions:
>
> 1. don't run matched disks from the same manufacturer and lot
> 2. watch disk temperature
> 3. watch smartmon for indications of aging
> 4. replace disks before they die
> 5. use your replacements as an opportunity to get your pairs staggered
> 6. have backups that at minimum are ping-ponged, current, and physically
> offline
> 7. goto #1...

In most cases this is not a case of simultaneous failure due to common 
disk wear or defects, or power supply events, or controller problems. In 
most cases of apparent simultaneous failure Disk 2 has a bad sector that 
has never been written to. Such a sector can remain undisturbed for the 
life of the disk, or until the RAID software attempts to sync with another 
disk. When Disk 1 fails (and is noticed by the RAID software) and is 
replaced the sync starts copying Disk 2 to the new Disk 1 and runs until 
the bad sector on Disk 2 is encountered, at which point it announces the 
fact that Disk 2 has failed. But it didn't fail during the sync - it was 
probably bad from day 1, and if written to would have been remapped 
transparently to the user and the Raid software. But sync doesn't write 
before reading. The only good thing is that data can still be read in 
degraded mode, and copied to another disk.

This is why simultaneous failures are so common in practice, even though 
statistically they should be quite rare.

Daniel Feenberg

>
>
> -kb
>
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