Without read ahead set, seq io performance will be lower than random..This is because Ceph IO path is serialized on a PG and for seq case there is more chance
to hit the same PG for consecutive IOs..
Thanks & Regards
Somnath
From: ceph-users [mailto:ceph-users-bounces@xxxxxxxxxxxxxx]
On Behalf Of min fang
Sent: Tuesday, February 02, 2016 11:34 PM
To: Mark Nelson
Cc: ceph-users
Subject: Re: [ceph-users] ceph random read performance is better than sequential read?
thanks, what I am talking is IOPs, which also reflect the bandwidth in fio testing. I did not set readahead parameter, I will try. One more question here is, for only 4k write or read IO random pattern, comparing
with using a single bigger rbd image, will performance(IOPS) be improved if I distribute IOs to a virtual volume consisted by multiple rbd images in a stripe way?
Thanks.
2016-02-02 22:41 GMT+08:00 Mark Nelson <mnelson@xxxxxxxxxx>:
If testing with fio and librbd, you may also find that increasing the thresholds for RBD readahead will help significantly. Specifically, set "rbd readahead disable after bytes" to 0 so rbd readahead stays enabled. In most cases with
buffered reads on a real client volume, rbd readahead isn't necessary, but with fio and the librbd engine this can make a big difference, especially with newstore.
Mark
On 02/02/2016 07:29 AM, Wade Holler wrote:
Could you share the fio command and your read_ahead_kb setting for the
OSD devices ? "performance is better" is a little too general. I
understand that we usually mean higher IOPS or higher aggregate
throughput when we say performance is better. However, application
random read performance "generally" implies an interest in lower latency
- which of course is much more involved from a testing perspective.
Cheers
Wade
On Tue, Feb 2, 2016 at 7:28 AM min fang <louisfang2013@xxxxxxxxx
<mailto:louisfang2013@xxxxxxxxx>> wrote:
Hi, I did a fio testing on my ceph cluster, and found ceph random
read performance is better than sequential read. Is it true in your
stand?
Thanks.
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