[sheepdog-users] Object cache (writethru/writeback) on gateway?

Liu Yuan namei.unix at gmail.com
Fri Jul 6 08:47:21 CEST 2012

On 07/06/2012 02:19 AM, icez network wrote:
> I'm using a single instance sheepdog on a RAID10 + Flashcache
> (https://github.com/facebook/flashcache/) and is interesting in the
> object caching implementation on sheepdog.
> So may I ask if thing like this can occurs in sheep
> - run each sheep daemon for each disk (4 disks in my setup) on each node
> without RAID.
> - run gateway and point to SSD mountpoint.
> - run QEMU on the same machine (for some node) and connect to gateway.

Yes you can.

> So the caching will store within gateway path (Am I guessing right?).
> I'm thinking that with SSD performance, it should boost I/O performance
> by large margin.
> As from the above design, may i ask few questions
> - Does object caching managed by gateway and store on the SSD?

Yes, object cache is only managed by sheep daemon which your VM
connected to, not QEMU. If you point sheep store to SSD, object cache is
stored on SSD.

Well, we can even add an option to sheep to specify where to store
object cache if useful.

> - The behavior of gateway if cache is filling out all spaces on the SSD.
> For example I'm using 120GB SSD which seems enough for general use but
> there's some excessive write on the QEMU for backing up their data once
> a day which can write a lot of data into it and it may exceed the SSD
> capacity. Will it bypass the write to the backend if cache write fail
> occurs?

Object cache automatic reclaim is under development by Levin Li. When
this finished (in weeks), we can take advantage of object cache even
with a small space transparently like kernel's page cache.


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