Bcache: Caching beyond just RAM [LWN.net]

Bcache: Caching beyond just RAM

Posted Jul 8, 2010 3:23 UTC (Thu) by koverstreet (✭ supporter ✭, #4296)
In reply to: Bcache: Caching beyond just RAM by NightMonkey
Parent article: Bcache: Caching beyond just RAM

Aside from tmpfs not being a block device, yes. I don't know that using hard drives as cache would ever be worthwhile - it's written with the assumption that random reads are fast, if you were to try that you'd want it to try to have it cache decent sized chunks of contiguous data (on cache miss, always read 64 kb or so in at a time). But it would certainly work.

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Bcache: Caching beyond just RAM

Posted Jul 8, 2010 3:35 UTC (Thu) by dlang (guest, #313) [Link] (2 responses)

a local magnetic drive may be faster than a network drive

Bcache: Caching beyond just RAM

Posted Jul 8, 2010 11:13 UTC (Thu) by ewan (guest, #5533) [Link] (1 responses)

And a small fast disk system (e.g. striped 15k SAS disks) may be a useful cache over a big slow one (e.g. a huge RAID of 5400rpm SATA disks).

I wonder if bcache can be stacked? So you could have an SSD over some SAS disks over a SATA RAID?

Bcache: Caching beyond just RAM

Posted Jul 8, 2010 11:24 UTC (Thu) by koverstreet (✭ supporter ✭, #4296) [Link]

There's no way to do that now - all the cache devices in the system are pooled together and used symmetrically - but it is something I've been considering down the road. It might not be all that much more work, I'm just curious if the performance would justify it. Most people's working set just isn't all that huge, and also the big problem with your big slow raid6 is random writes, which bcache should be able to mostly eliminate. But we'll see.

Bcache: Caching beyond just RAM

Posted Jul 8, 2010 3:42 UTC (Thu) by NightMonkey (subscriber, #23051) [Link] (1 responses)

I see. It might be interesting to use with a hard drive (or RAID 0 set) as a write-behind cache, once that is implemented. Or perhaps in front of a high-latency DRBD device. Some interesting possibilities! :)

Bcache: Caching beyond just RAM

Posted Jul 8, 2010 15:40 UTC (Thu) by martinfick (subscriber, #4455) [Link]

Why would you want it in front of a DRBD device? If you really want to sacrifice reliability for performance with a DRBD device, simply use protocol B or C, it will likely be more reliable.