Bug 1367891

Summary:	rbd cache does not switch to writeback mode after first flush
Product:	Red Hat Ceph Storage	Reporter:	Jason Dillaman <jdillama>
Component:	RBD	Assignee:	Jason Dillaman <jdillama>
Status:	CLOSED ERRATA	QA Contact:	ceph-qe-bugs <ceph-qe-bugs>
Severity:	medium	Docs Contact:
Priority:	unspecified
Version:	2.0	CC:	ceph-eng-bugs, kdreyer, nlevine, vakulkar, vumrao
Target Milestone:	rc
Target Release:	2.0
Hardware:	Unspecified
OS:	Unspecified
Whiteboard:
Fixed In Version:	RHEL: ceph-10.2.2-39.el7cp Ubuntu: ceph_10.2.2-31redhat1xenial	Doc Type:	If docs needed, set a value
Doc Text:		Story Points:	---
Clone Of:		Environment:
Last Closed:	2016-09-15 11:18:01 UTC	Type:	Bug
Regression:	---	Mount Type:	---
Documentation:	---	CRM:
Verified Versions:		Category:	---
oVirt Team:	---	RHEL 7.3 requirements from Atomic Host:
Cloudforms Team:	---	Target Upstream Version:
Bug Depends On:
Bug Blocks:	1363819

Description Jason Dillaman 2016-08-17 17:45:19 UTC

Description of problem:
With the default config option of 'rbd_cache_writethrough_until_flush' set to true, librbd waits for a flush request before enabling the writeback cache. However, librbd is not enabling the writeback cache even after a flush is received.

This results in a several performance penalty for librbd-backed VMs.

Version-Release number of selected component (if applicable):


How reproducible:
100%

Steps to Reproduce:
1. Compare IOPS between a VM configured with 'rbd_cache_writethrough_until_flush=false' and one with 'rbd_cache_writethrough_until_flush=true'

Actual results:
VM tests using 'rbd_cache_writethrough_until_flush=false' will have substantially higher random write IOPS compared to without due to the fact that the cache is stuck in writethrough mode.

Expected results:
Writeback cache should be enabled after first flush is received.

Additional info:

Comment 3 Jason Dillaman 2016-08-17 17:46:17 UTC

Upstream pull request: https://github.com/ceph/ceph/pull/10762

Comment 7 Vasu Kulkarni 2016-08-24 20:46:08 UTC

I can also add one system level test in fio with and without that option.

Comment 13 Vasu Kulkarni 2016-09-09 19:14:59 UTC

Verified(sanity only)

Comment 16 errata-xmlrpc 2016-09-15 11:18:01 UTC

Since the problem described in this bug report should be
resolved in a recent advisory, it has been closed with a
resolution of ERRATA.

For information on the advisory, and where to find the updated
files, follow the link below.

If the solution does not work for you, open a new bug report.

https://rhn.redhat.com/errata/RHBA-2016-1898.html