Bcache: Caching beyond just RAM [LWN.net]

Bcache: Caching beyond just RAM

Posted Jul 8, 2010 17:34 UTC (Thu) by sflintham (guest, #47422)
Parent article: Bcache: Caching beyond just RAM

This approach of intercepting bio requests in the background allows us to start and stop caching on the fly, to add and remove cache devices, and to boot with or without bcache

Am I right in thinking this will only be possible once the checksumming feature mentioned at the bottom of the article is in place? Otherwise, what would stop the cache returning an out-of-date copy of a block modified while it was offline?

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Bcache: Caching beyond just RAM

Posted Jul 8, 2010 23:16 UTC (Thu) by koverstreet (✭ supporter ✭, #4296) [Link]

Checksumming won't save you here - what's it supposed to compared the checksum against? If you fill up your cache, turn of caching, write some data, and then turn caching back on without telling bcache about it... well, computers can't always save you from rm -rf either.

I am looking into invalidating the cache contents if a filesystem was already opened read write, or perhaps checking if the first page or so matches what it previously had - this would catch it provided the filesystem superblock changed. But the real performance gains are to be had with write behind caching, and none of this really matters there, since the cached device is now inconsistent if you use it without the cache. By the time bcache saw the cache was out of date it'd be too late to do anything.