[ceph-users] Can CephFS Kernel Client Not Read & Write at the Same Time?

Gregory Farnum gfarnum at redhat.com
Thu Mar 7 16:15:11 PST 2019


In general, no, this is not an expected behavior.

My guess would be that something odd is happening with the other clients
you have to the system, and there's a weird pattern with the way the file
locks are being issued. Can you be more precise about exactly what workload
you're running, and get the output of the session list on your MDS while
doing so?
-Greg

On Wed, Mar 6, 2019 at 9:49 AM Andrew Richards <
andrew.richards at keepertech.com> wrote:

> We discovered recently that our CephFS mount appeared to be halting reads
> when writes were being synched to the Ceph cluster to the point it was
> affecting applications.
>
> I also posted this as a Gist with embedded graph images to help
> illustrate:
> https://gist.github.com/keeperAndy/aa80d41618caa4394e028478f4ad1694
>
> The following is the plain text from the Gist.
>
> First, details about the host:
>
> ````
>     $ uname -r
>     4.16.13-041613-generic
>
>     $ egrep 'xfs|ceph' /proc/mounts
>     192.168.1.115:6789,192.168.1.116:6789,192.168.1.117:6789:/ /cephfs
> ceph rw,noatime,name=cephfs,secret=<hidden>,rbytes,acl,wsize=16777216 0 0
>     /dev/mapper/tst01-lvidmt01 /rbd_xfs xfs
> rw,relatime,attr2,inode64,logbsize=256k,sunit=512,swidth=1024,noquota 0 0
>
>     $ ceph -v
>     ceph version 12.2.5 (cad919881333ac92274171586c827e01f554a70a)
> luminous (stable)
>
>     $ cat /proc/net/bonding/bond1
>     Ethernet Channel Bonding Driver: v3.7.1 (April 27, 2011)
>
>     Bonding Mode: adaptive load balancing
>     Primary Slave: None
>     Currently Active Slave: net6
>     MII Status: up
>     MII Polling Interval (ms): 100
>     Up Delay (ms): 200
>     Down Delay (ms): 200
>
>     Slave Interface: net8
>     MII Status: up
>     Speed: 10000 Mbps
>     Duplex: full
>     Link Failure Count: 2
>     Permanent HW addr: e4:1d:2d:17:71:e1
>     Slave queue ID: 0
>
>     Slave Interface: net6
>     MII Status: up
>     Speed: 10000 Mbps
>     Duplex: full
>     Link Failure Count: 1
>     Permanent HW addr: e4:1d:2d:17:71:e0
>     Slave queue ID: 0
>
> ````
>
> We had CephFS mounted alongside an XFS filesystem made up of 16 RBD images
> aggregated under LVM as our storage targets. The link to the Ceph cluster
> from the host is a mode 6 2x10GbE bond (bond1 above).
>
> We started capturing network counters from the Ceph cluster connection
> (bond1) on the host using ifstat at its most granular setting of 0.1
> (sampling every tenth of a second). We then ran various overlapping read
> and write operations in separate shells on the same host to obtain samples
> of how our different means of accessing Ceph handled this. We converted our
> ifstat output to CSV and insterted it into a spreadsheet to visualize the
> network activity.
>
> We found that the CephFS kernel mount did indeed appear to pause ongoing
> reads when writes were being flushed from the page cache to the Ceph
> cluster.
>
> We wanted to see if we could make this more pronounced, so we added a
> 6Gb-limit tc filter to the interface and re-ran our tests. This yielded
> much lengthier delay periods in the reads while the writes were more slowly
> flushed from the page cache to the Ceph cluster.
>
> A more restrictive 2Gbit-limit tc filter produced much lengthier delays of
> our reads as the writes were synched to the cluster.
>
> When we tested the same I/O on the RBD-backed XFS file system on the same
> host, we found a very different pattern. The reads seemed to be given
> priority over the write activity, but the writes were only slowed, they
> were not halted.
>
> Finally we tested overlapping SMB client reads and writes to a Samba share
> that used the userspace libceph-based VFS_Ceph module to produce the share.
> In this case, while raw throughput was lower than that of the kernel, the
> reads and writes did not interrupt each other at all.
>
> Is this expected behavior for the CephFS kernel drivers? Can a CephFS
> kernel client really not read and write to the file system simultaneously?
>
> Thanks,
> Andrew Richards
> Senior Systems Engineer
> Keeper Technology, LLC
>
> _______________________________________________
> ceph-users mailing list
> ceph-users at lists.ceph.com
> http://lists.ceph.com/listinfo.cgi/ceph-users-ceph.com
>
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