Hi Simon
Do your workload has lots of RAW? Since Ceph has RW lock in each object, so if you have a write to RBD and the following read happen to hit the same object,
the latency will be higher.
Another possibility is the OSD op_wq, it’s a priority queue but read and write have same priority here and waiting Op_threads to serve them. So even through
you use deadline in the storage node ,but in general ,not making much difference. Our BKM usually goes to set osd op threads to 20
If you could use ceph performance counter to identify if there is any throttle has long wait time, together with your HW configuration and IOSTAT data,
that should be helpful for other to understand your case and help you.
Xiaoxi
From: ceph-users [mailto:ceph-users-bounces@xxxxxxxxxxxxxx]
On Behalf Of Nick Fisk
Sent: Saturday, November 1, 2014 4:26 AM
To: ceph-users@xxxxxxxxxxxxxx
Subject: Re: [ceph-users] prioritizing reads over writes
I think I might have to step out on this one, it sounds like you have all the basics covered
for best performance and I can’t think what else to suggest.
Sorry I couldn’t be of more help.
From: ceph-users [mailto:ceph-users-bounces@xxxxxxxxxxxxxx]
On Behalf Of Xu (Simon) Chen
Sent: 31 October 2014 20:15
To: Nick Fisk
Cc: ceph-users@xxxxxxxxxxxxxx
Subject: Re: [ceph-users] prioritizing reads over writes
We have SSD journals, backend disks are actually on SSD-fronted bcache devices in writeback mode. The client VMs have rbd cache enabled too...
On Fri, Oct 31, 2014 at 4:07 PM, Nick Fisk <nick@xxxxxxxxxx> wrote:
Hmmm, it sounds like you are just saturating the spindles to the point that latency starts to climb to unacceptable levels. The problem being that no matter how much
tuning you apply, at some point the writes will have to start being put down to the disk and at that point performance will suffer.
Do your OSD’s have SSD journals? In storage, normally adding some sort of writeback cache (in Ceph’s case Journals) help to lessen the impact of writes by asorbing
bursts of writes and by coalescing writes into a more sequential pattern to the underlying disks.
From: ceph-users [mailto:ceph-users-bounces@xxxxxxxxxxxxxx]
On Behalf Of Xu (Simon) Chen
Sent: 31 October 2014 19:51
To: Nick Fisk
Cc: ceph-users@xxxxxxxxxxxxxx
Subject: Re: [ceph-users] prioritizing reads over writes
I am already using deadline scheduler, with the default parameters:
I remember tuning them before, didn't make a great difference.
Hi Simon,
Have you tried using the Deadline scheduler on the Linux nodes? The deadline scheduler prioritises reads over writes. I believe it tries to service all reads within
500ms whilst writes can be delayed up to 5s.
I don’t the exact effect Ceph will have over the top of this, but this would be the first thing I would try.
Nick
From: ceph-users [mailto:ceph-users-bounces@xxxxxxxxxxxxxx]
On Behalf Of Xu (Simon) Chen
Sent: 31 October 2014 19:37
To: ceph-users@xxxxxxxxxxxxxx
Subject: [ceph-users] prioritizing reads over writes
Hi all,
My workload is mostly writes, but when the writes reach a certain throughput (iops wise not much higher) the read throughput would tank. This seems to be impacting my VMs' responsiveness overall. Reads would recover after write throughput
drops.
Is there any way to prioritize read over write, or at least guarantee a certain level of aggregated read throughput in a cluster?
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