O_DIRECT read of sector 0 [LWN.net]

O_DIRECT read of sector 0

Posted Jul 9, 2010 15:37 UTC (Fri) by wstearns (guest, #4102)
In reply to: Bcache: Caching beyond just RAM by eludias
Parent article: Bcache: Caching beyond just RAM

Since bcache can only cache individual filesystems and I assume your actual filesystems are on partitions (as opposed to a single filesystem covering the entire drive), bcache won't be caching sector 0 as that's not part of a filesystem.

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O_DIRECT read of sector 0

Posted Jul 10, 2010 10:45 UTC (Sat) by eludias (guest, #4058) [Link] (1 responses)

I was under the impression that bcache cached block devices (hence the name ), and not filesystems. But you have a good point: if I would cache the partition and not the whole device, I might be able to circumvent it by reading outside the partition.

O_DIRECT read of sector 0

Posted Jul 10, 2010 21:13 UTC (Sat) by wstearns (guest, #4102) [Link]

You're correct; bcache does cache block devices. I phrased it poorly; the idea I failed to get across was that the filesystems we'd normally submit to bcache to cache are on individual partitions (which linux treats as block devices) as opposed to the entire drive, allowing you to bypass bcache on sector 0 and still cache the partitions.