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On Tue, Jun 24, 2014 at 2:34 PM, Kai Krakow <hurikhan77@×××××.com> wrote: |
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> I'm not sure if multiple partitions can share the same cache device |
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> partition but more or less that's it: Initialize bcache, then attach your |
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> backing devices, then add those bcache devices to your btrfs. |
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Ah, if you are stuck with one bcache partition per cached device then |
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that will be fairly painful to manage. |
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> Yes, it will write double the data to the cache then - but only if btrfs |
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> also did actually read both copies (which it probably does not because it |
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> has checksums and does not need to compare data, and lets just ignore the |
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> case that another process could try to read the same data from the other |
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> raid member later, that case should become optimized-out by the OS cache). |
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I didn't realize you were proposing read caching only. If you're only |
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caching reads then obviously that is much safer. I think with btrfs |
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in raid1 mode with only two devices you can tell it to prefer a |
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particular device for reading in which case you could just bcache that |
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drive. It would only read from the other drive if the cache failed. |
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However, I don't think btrfs lets you manually arrange drives into |
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array-like structures. It auto-balances everything which is usually a |
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plus, but if you have 30 disks you can't tell it to treat them as 6x |
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5-disk RAID5s vs one 30-disk raid5 (I think). |
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Rich |
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