Bcache and filesystem recovery [LWN.net]

Bcache and filesystem recovery

Posted Jul 8, 2010 16:54 UTC (Thu) by abacus (guest, #49001)
Parent article: Bcache: Caching beyond just RAM

An important issue has not been discussed in the above article, and that is how file system recovery works when using Bcache. Does Bcache honor the write ordering requirements communicated by filesystems such that e.g. journal-based filesystem recovery still works ?

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Bcache and filesystem recovery

Posted Jul 9, 2010 2:15 UTC (Fri) by koverstreet (✭ supporter ✭, #4296) [Link]

It doesn't handle barriers just yet. That's coming soon, it's needed for write behind caching.

This isn't a correctness issue, if you don't have barriers you just have to wait for writes to complete before you start another to guarantee ordering. Barriers have historically taken awhile to add to things like raid, lvm and drbd, bcache is nothing special.

There is a bug in that if you're running bcache on something that supports barriers, bcache won't indicated that they're no longer supported... I haven't worried much about that since you can disable them in the filesystem and it's only till I get barriers implemented in bcache.