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a plea for non-identical mirrors

When I set up my test environment earlier this year, I put two Samsung HD103UJ 1TB drives in the main file server, and configured them as a RAID-1 software mirror through mdadm. Just months later, smartd was spewing out messages about both drives: Device: /dev/sda, 8 Currently unreadable (pending) sectors.

That got me very worried... I replaced one of the drives with a WDC WD1000FYPS-01ZKB0, and rebuilt the mirror. Actually, I built new mdadm mirrors from smaller partitions to lessen the risk of a two-drive badblock scenario. Regardless, my data got moved, so now everything gets stored on two different drives. Different makes, manufacturers, models. Same size though.

The problem is, I can't RMA the drive. It's still in warranty, but when running badblocks -w on the drive, it shows no problems whatsoever. Moreover, the SMART status has gone back to: Current_Pending_Sector 0. So not only is the problem undetectable by badblocks, it even makes it go away in the SMART counters where it was visible in the first place.
First I thought that the problematic sectors got reallocated, but the SMART info suggests otherwise: Reallocated_Event_Count equals 0. If you want to look at your own drives, grep for _Pending_ and Reallocated_ in the smartctl -a output.

My conclusion: it was a very, very bad idea to make a mirror out of two identical drives (and aggravating the fact, they were the cheapest ones, too). Please remember: pick different drives to make up a mirror. A mirror image needs to be identical, but having it on identical media is a bad, bad thing.

And to round up, my final thoughts: how bad is a non-zero "Current_Pending_Sector" count, really ? And why can it go down without reallocation ?

Side note: I'm not saying Samsung drives are bad, I have 3 Samsung HD300LJ in a RAID-1 setup that are doing fine after more than 2 years of 24x7 service. But buying their 1TB drives left a sour aftertaste...


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