[Gluster-devel] I/O performance

[Xavi Hernandez]

On Fri, Feb 1, 2019 at 1:51 PM Xavi Hernandez <xhernandez at redhat.com> wrote:

> On Fri, Feb 1, 2019 at 1:25 PM Poornima Gurusiddaiah <pgurusid at redhat.com>
> wrote:
>
>> Can the threads be categorised to do certain kinds of fops?
>>
>
> Could be, but creating multiple thread groups for different tasks is
> generally bad because many times you end up with lots of idle threads which
> waste resources and could increase contention. I think we should only
> differentiate threads if it's absolutely necessary.
>
>
>> Read/write affinitise to certain set of threads, the other metadata fops
>> to other set of threads. So we limit the read/write threads and not the
>> metadata threads? Also if aio is enabled in the backend the threads will
>> not be blocked on disk IO right?
>>
>
> If we don't block the thread but we don't prevent more requests to go to
> the disk, then we'll probably have the same problem. Anyway, I'll try to
> run some tests with AIO to see if anything changes.
>

I've run some simple tests with AIO enabled and results are not good. A
simple dd takes >25% more time. Multiple parallel dd take 35% more time to
complete.

Xavi


> All this is based on the assumption that large number of parallel read
>> writes make the disk perf bad but not the large number of dentry and
>> metadata ops. Is that true?
>>
>
> It depends. If metadata is not cached, it's as bad as a read or write
> since it requires a disk access (a clear example of this is the bad
> performance of 'ls' in cold cache, which is basically metadata reads). In
> fact, cached data reads are also very fast, and data writes could go to the
> cache and be updated later in background, so I think the important point is
> if things are cached or not, instead of if they are data or metadata. Since
> we don't have this information from the user side, it's hard to tell what's
> better. My opinion is that we shouldn't differentiate requests of
> data/metadata. If metadata requests happen to be faster, then that thread
> will be able to handle other requests immediately, which seems good enough.
>
> However there's one thing that I would do. I would differentiate reads
> (data or metadata) from writes. Normally writes come from cached
> information that is flushed to disk at some point, so this normally happens
> in the background. But reads tend to be in foreground, meaning that someone
> (user or application) is waiting for it. So I would give preference to
> reads over writes. To do so effectively, we need to not saturate the
> backend, otherwise when we need to send a read, it will still need to wait
> for all pending requests to complete. If disks are not saturated, we can
> have the answer to the read quite fast, and then continue processing the
> remaining writes.
>
> Anyway, I may be wrong, since all these things depend on too many factors.
> I haven't done any specific tests about this. It's more like a
> brainstorming. As soon as I can I would like to experiment with this and
> get some empirical data.
>
> Xavi
>
>
>> Thanks,
>> Poornima
>>
>>
>> On Fri, Feb 1, 2019, 5:34 PM Emmanuel Dreyfus <manu at netbsd.org wrote:
>>
>>> On Thu, Jan 31, 2019 at 10:53:48PM -0800, Vijay Bellur wrote:
>>> > Perhaps we could throttle both aspects - number of I/O requests per
>>> disk
>>>
>>> While there it would be nice to detect and report  a disk with lower than
>>> peer performance: that happen sometimes when a disk is dying, and last
>>> time I was hit by that performance problem, I had a hard time finding
>>> the culprit.
>>>
>>> --
>>> Emmanuel Dreyfus
>>> manu at netbsd.org
>>> _______________________________________________
>>> Gluster-devel mailing list
>>> Gluster-devel at gluster.org
>>> https://lists.gluster.org/mailman/listinfo/gluster-devel
>>>
>>
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