Change default value of db prealloc

Registered by John Dickinson on 2012-05-22

We don't like changing the default behavior of Swift unless there's a
really good reason and, up until now, I've tried doing this with this
new db_preallocation setting.

For clusters with dedicated account/container servers that usually
have fewer disks overall but SSD for speed, having db_preallocation
on will gobble up disk space quite quickly and the fragmentation it's
designed to fight isn't that big a speed impact to SSDs anyway.

For clusters with account/container servers spread across all servers
along with object servers usually having standard disks for cost,
having db_preallocation off will cause very fragmented database files
impacting speed, sometimes dramatically.

Weighing these two negatives, it seems the second is the lesser evil.
The first can cause disks to fill up and disable the cluster. The
second will cause performance degradation, but the cluster will still
function.

Furthermore, if just one piece of code that touches all databases
runs with db_preallocation on, it's effectively on for the whole
cluster. We discovered this most recently when we finally configured
everything within the Swift codebase to have db_preallocation off,
only to find out Slogging didn't know about the new setting and so
ran with it on and starting filling up SSDs.

So that's why I'm proposing this change to the default behavior.

We will definitely need to post a prominent notice of this change
with the next release.

Blueprint information

Status:
Complete
Approver:
None
Priority:
Essential
Drafter:
None
Direction:
Approved
Assignee:
None
Definition:
Approved
Series goal:
Accepted for folsom
Implementation:
Implemented
Milestone target:
milestone icon 1.5.0
Started by
John Dickinson on 2012-05-22
Completed by
John Dickinson on 2012-05-24

Related branches

Sprints

Whiteboard

(?)

Work Items

This blueprint contains Public information 
Everyone can see this information.

Subscribers

No subscribers.