On 4 Feb 2020, at 16:14, Samy Ascha <samy@xel.nl> wrote:
On 2 Feb 2020, at 12:45, Patrick Donnelly <pdonnell@redhat.com> wrote:
On Wed, Jan 29, 2020 at 1:25 AM Samy Ascha <samy@xel.nl> wrote:
Hi!
I've been running CephFS for a while now and ever since setting it up, I've seen unexpectedly large write i/o on the CephFS metadata pool.
The filesystem is otherwise stable and I'm seeing no usage issues.
I'm in a read-intensive environment, from the clients' perspective and throughput for the metadata pool is consistently larger than that of the data pool.
For example:
# ceph osd pool stats pool cephfs_data id 1 client io 7.6 MiB/s rd, 19 KiB/s wr, 404 op/s rd, 1 op/s wr
pool cephfs_metadata id 2 client io 338 KiB/s rd, 43 MiB/s wr, 84 op/s rd, 26 op/s wr
I realise, of course, that this is a momentary display of statistics, but I see this unbalanced r/w activity consistently when monitoring it live.
I would like some insight into what may be causing this large imbalance in r/w, especially since I'm in a read-intensive (web hosting) environment.
The MDS is still writing its journal and updating the "open file table". The MDS needs to record certain information about the state of its cache and the state issued to clients. Even if the clients aren't changing anything. (This is workload dependent but will be most obvious when clients are opening files _not_ in cache already.)
-- Patrick Donnelly, Ph.D. He / Him / His Senior Software Engineer Red Hat Sunnyvale, CA GPG: 19F28A586F808C2402351B93C3301A3E258DD79D
Hi Patrick,
Thanks for this extra information.
I should be able to confirm this by checking network traffic flowing from the MDSes to the OSDs, and compare it to what's coming in from the CephFS clients.
I'll report back when I have more information on that. I'm a little caught up in other stuff right now, but I wanted to just acknowledge your message.
Samy
Hi! I've confirmed that the write IO to the metadata pool is coming form active MDSes. I'm experiencing very poor write performance on clients and I would like to see if there's anything I can do to optimise the performance. Right now, I'm specifically focussing on speeding up this use case: In CephFS mounted dir: $ time unzip -q wordpress-seo.12.9.1.zip real 0m47.596s user 0m0.218s sys 0m0.157s On RBD mount: $ time unzip -q wordpress-seo.12.9.1.zip real 0m0.176s user 0m0.131s sys 0m0.045s The difference is just too big. I'm having real trouble finding a good reference to check my setup for bad configuration etc. I have network bandwidth, RAM and CPU to spare, but I'm unsure on how to put it to work to help my case. Thanks a lot, Samy