Re: [ANNOUNCE] Ramback: faster than a speeding bullet
- From: Lars Marowsky-Bree <lmb@xxxxxxx>
- Date: Tue, 11 Mar 2008 12:23:29 +0100
On 2008-03-11T03:14:40, Daniel Phillips <phillips@xxxxxxxxx> wrote:
You get duplicated blocks though. But yes, I agree - write-backs to theI disagree with your claim of "too unreliable". If the UPS power does
disk must be ordered, other it's going to be too unreliable in practice.
not fail before flushing completes, it is perfectly reliable. Perhaps
you need a belt to go with your suspenders?
Daniel, I'm not saying you don't have a good thing here. Just that for
backing large filesystems, the risk of having to run a full fsck and
finding inconsistent metadata is pretty serious.
If I always assume a reliable shutdown - UPS protected, no crashes, etc
- you're right, but at least my real world has other failure scenarios
as well. In fact, the most common reason for unorderly shutdowns are
kernel crashes, not power failures in my experience.
So "perfectly reliable if UPS power does not fail" seems a bit over the
top.
As I wrote earlier, you cannot have optimal writeback speed and ordering
at the same time.
No disagreement here. The question would be how large the performance
difference is.
I can see eventually implementing some kind of ordered
writeback mode where completion is signalled to the application before
writeback completes. You then get to choose between fastest flush and
most paranoid ordering. I guess everybody will choose fastest flush,
but I will be happy to accept your patch to see which they actually
choose.
I was trying to prod you into writing the ordered flushing. Maybe
claiming it is too hard will do the trick? ;-)
Seriously, I guess it depends on the workload you want to host.
If you think this is like replication then you have the wrong ideaYes you may need to throttle in the specific case of having too manyYou could switch from a journal like the above to a bitmap when this
copies of pages sitting in the queue - but surely that would be the set of
pages that are written but not yet committed from a previous store
barrier ?
overrun occurs. (Typical problem in replication.) SteelEye holds a
patent on that though, as far as I know.
about what is going on. This is a cache consistency algorithm, not
a replication algorithm.
I see the differences, but I also see the similarities. What you're
doing can also be thought of as replicating from an instant IO store
(local memory) to a high latency, low bandwidth copy (the disk)
asynchronously.
Both obviously need to preserve consistency, the question is whether to
achieve transactional (ordered) consistency or not.
Regards,
Lars
--
Teamlead Kernel, SuSE Labs, Research and Development
SUSE LINUX Products GmbH, GF: Markus Rex, HRB 16746 (AG Nürnberg)
"Experience is the name everyone gives to their mistakes." -- Oscar Wilde
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