-Re: File channel performance on a single disk is poor
Juhani Connolly 2012-07-09, 06:14
Hi, thanks for your input.
On 07/09/2012 02:42 PM, Arvind Prabhakar wrote:
> > It's certainly one possible solution to the issue, though I do
> > believe that the current one could be made more friendly
> > towards single disk access(e.g. batching writes to the disk
> > may well be doable and would be curious what someone
> > with more familiarity with the implementation thinks).
> The implementation of the file channel is that of a write ahead log,
> in that it serializes all the actions as they happen. Using these
> actions, it can reconstruct the state of the channel at anytime. There
> are two mutually exclusive transaction types it supports - a
> transaction consisting of puts, and one consisting of takes. It may be
> possible to use the heap to batch the puts and takes and serialize
> them to disk when the commit occurs.
> This approach will minimize the number of disk operations and will
> have an impact on the performance characteristics of the channel.
> Although it probably will improve performance, it is hard to tell for
> sure unless we test it out under load in different scenarios.
This does sound a lot better to me. I'm not sure if there is much demand
for restoring the state of an uncommitted set of puts/takes to a file
channel after restarting an agent? If the transaction wasn't completed
its current state is not really going to be important after a restart.
I'm really not familiar with WAL implementations, but is it not merely
enough to write the data to be committed before the commit
marker/informing of success? I don't think it is necessary to write each
piece as it comes in, so long as it is done before informing of
Another matter that I'm curious of is whether or not we actually need
separate files for the data and checkpoints... Can we not add a magic
header before each type of entry to differentiate, and thus guarantee
significantly more sequential access? What is killing performance on a
single disk right now is the constant seeks. The problem with this
though would be putting together a file format that allows quick seeking
through to the correct position, and rolling would be a lot harder. I
think this is a lot more difficult and might be more of a long term target.
> Arvind Prabhakar
> On Wed, Jul 4, 2012 at 3:33 AM, Juhani Connolly
> <[EMAIL PROTECTED]
> <mailto:[EMAIL PROTECTED]>> wrote:
> It looks good to me as it provides a nice balance between
> reliability and throughput.
> It's certainly one possible solution to the issue, though I do
> believe that the current one could be made more friendly towards
> single disk access(e.g. batching writes to the disk may well be
> doable and would be curious what someone with more familiarity
> with the implementation thinks).
> On 07/04/2012 06:36 PM, Jarek Jarcec Cecho wrote:
> We had connected discussion about this "SpillableChannel"
> (working name) on FLUME-1045 and I believe that consensus is
> that we will create something like that. In fact, I'm planning
> to do it myself in near future - I just need to prioritize my
> todo list first.
> On Wed, Jul 04, 2012 at 06:13:43PM +0900, Juhani Connolly wrote:
> Yes... I was actually poking around for that issue as I
> seeing it before. I had before also suggested a compound
> that would have worked like the buffer store in scribe,
> but general
> opinion was that it provided too many mixed configurations
> could make testings and verifying correctness difficult.
> On 07/04/2012 04:33 PM, Jarek Jarcec Cecho wrote:
> Hi Juhally,
> while ago I've filled jira FLUME-1227 where I've
> suggested creating some sort of SpillableChannel that