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Yo Rich! |
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On Fri, 21 Jun 2013 13:57:20 -0400 |
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Rich Freeman <rich0@g.o> wrote: |
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> In theory it could be done that way, but every |
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> raid1 implementation I've heard of makes writes to all drives |
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> (obviously), but reads from only a single drive (assuming it is |
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> correct). That means that read latency is greatly reduced since they |
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> can be split across two drives which effectively means two heads per |
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> "platter." |
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Yes, that is what I see in practice. A much reduced average read time. |
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And if you are really pressed for speed, add more stripes and get even |
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more speed. |
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> Also, raid1 typically does not include checksumming, so if |
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> there is a discrepancy between the drives there is no way to know |
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> which one is right. |
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Uh, not exactly correct. Remember each HDD has ECC for each sector. If |
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there is a read error the HDD will detecct the bad ECC and report the |
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error to the RAID1 hardware/software. Then RAID1 is smart enough to try |
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to read from the 2nd drive. |
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> With raid5 at least you can always correct |
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> discrepancies if you have all the disks |
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Not really. If 2 disks fail in an n+1 RAID5 you are out of luck. |
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Not as uncommon occurance as one might think. |
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> (though as Duncan pointed out |
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> in practice this only happens if you do an explicit scrub on mdadm). |
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Which you should be doing at least weekly. Otherwise you only find out |
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your disks have failed when you try to do a full copy or backup, and then |
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you likely have multiple failures and you are out of luck. |
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RGDS |
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GARY |
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--------------------------------------------------------------------------- |
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Gary E. Miller Rellim 109 NW Wilmington Ave., Suite E, Bend, OR 97701 |
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gem@××××××.com Tel:+1(541)382-8588 |
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