Bug 1788347

Summary:	reduce min_alloc_size to improve space usage for small objects
Product:	[Red Hat Storage] Red Hat Ceph Storage	Reporter:	Josh Durgin <jdurgin>
Component:	RADOS	Assignee:	Neha Ojha <nojha>
Status:	CLOSED ERRATA	QA Contact:	Manohar Murthy <mmurthy>
Severity:	high	Docs Contact:	Karen Norteman <knortema>
Priority:	high
Version:	4.0	CC:	agunn, ceph-eng-bugs, ceph-qe-bugs, dzafman, hyelloji, jbrier, kchai, kdreyer, knortema, nojha, tserlin
Target Milestone:	rc	Keywords:	Performance, UserExperience
Target Release:	4.1	Flags:	hyelloji: needinfo-
Hardware:	Unspecified
OS:	Unspecified
Whiteboard:
Fixed In Version:	ceph-14.2.8-3.el8, ceph-14.2.8-3.el7	Doc Type:	Bug Fix
Doc Text:	.Small objects and files in RADOS no longer use more space than required The Ceph Object Gateway and the Ceph file system (CephFS) stores small objects and files as individual objects in RADOS. Previously, objects smaller than BlueStore's default minimum allocation size (`min_alloc_size`) of 16 KB used more space than required. This happened because the earlier default value of BlueStore's `min_alloc_size` was 16 KB for solid state devices (SSDs). Currently, the default value of `min_alloc_size` for SSDs is 4 KB. This enables better use of space with no impact on performance.	Story Points:	---
Clone Of:		Environment:
Last Closed:	2020-05-19 17:31:40 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:
Embargoed:
Bug Depends On:
Bug Blocks:	1816167

Description Josh Durgin 2020-01-07 01:46:59 UTC

Description of problem:
RGW or CephFS store small objects and files as individual objects in rados. Objects smaller than bluestore's min_alloc_size (16k for ssd) use much more space than required.

min_alloc_size of 4k has been shown to have no detriment to performance for SSD.
Use this as the default for SSDs.

How reproducible:
100%

Steps to Reproduce:
1. Write many 4k files to cephfs.
2. Run 'ceph osd df'

Actual results:
Raw space used is >16kib per object.

Expected results:
Raw space used is closer to 4kib per object.

Comment 2 Josh Durgin 2020-03-23 20:42:12 UTC

included in 14.2.8 rebase

Comment 13 errata-xmlrpc 2020-05-19 17:31:40 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://access.redhat.com/errata/RHSA-2020:2231