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On 27 October 2011 20:35, Mark Knecht <markknecht@×××××.com> wrote: |
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> On Thu, Oct 27, 2011 at 11:41 AM, Jarry <mr.jarry@×××××.com> wrote: |
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>> Hi, perhaps someone could explain this to me: |
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>> |
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>> I have bouth two the same hard-drives. The same model |
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>> (Hitachi HUA722050CLA330), the same firmware (JP20A3EA), |
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>> the same size (500GB). Well, not exactly. Both hdparm |
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>> and fdisk report different number of sectors (976771055 |
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>> versus 976773168). Although not a big difference, yet |
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>> I expected them to be exactly the same (want to use |
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>> them for raid1). |
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>> |
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>> So how is it possible they do not have the same number |
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>> of sectors? I have bought them from one supplier, even |
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>> their serial numbers are very close (only the last 2 |
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>> characters out of 24 are different)... |
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>> |
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>> Jarry |
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> |
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> Maybe one has some stuff mapped out due to bad blocks found during |
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> manufacturing or something like that? Not sure what it will tell you |
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> but have you run smartctl on the drives and looked around at what they |
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> tell you to find any differences? |
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> |
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> - Mark |
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|
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During normal operation, if a bad block is detected, that sector is |
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marked as 'bad', and a one of the free sectors (which are additional |
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to your totals) is allocated to replace it. This is called |
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Re-Allocating Sectors, and according to the Google paper[1], which |
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seems to be the only authoritative (non-marketing, non |
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industry-funded) source on hard-drive failure, re-allocated sectors |
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are indicative of impending drive failure. You can check your |
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Re-Allocated sector count using smartmontools (but I recommend that |
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you try gsmartcontrol in sunrise, which makes life easier). |
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|
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This is made more complicated by the fact that if bad sectors (below a |
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manufacturing threshold) are detected in factory testing, they will |
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re-allocate them, and reset the SMART counter to Zero (the drive _is_ |
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brand new after all!). Thus, you can buy two of the exact same model |
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of drive, and yet have different numbers of available sectors. |
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|
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It is also possible that something entirely different is at play. |
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|
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[1] labs.google.com/papers/disk_failures.pdf |
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