When ceph synchronizes journal to disk?

[Xing Lin <xinglin@cs.utah.edu>]

Hi,

There were some discussions about this before on the mailing list but I 
am still confused with this. I thought Ceph would flush data from the 
journal to disk when either the journal is full or when the time to do 
synchronization is due. In my test experiment, I used 24 osds(one osd 
for each disk). I used a 10 GB tmpfs file as the journal disk for each 
osd. Then for testing, I delayed the synchronization between the journal 
and disk on purpose. I increased the 'journal min sync interval' to be 
60 s and 'journal max sync interval' to be 300 s. Then I created a rbd 
and then started a 4M sequential write workload with fio for 30 seconds. 
I was expecting that no IO should happen to disks, unless we have filled 
240 GB data (10G*24). However, 'iostat' showed there was data
started to be written into disks (at about 20 MB/s per disk), right 
after I started the sequential workload. Could someone help to explain 
this situation? Thanks,

I am running 0.48.2. The related configuration is as follows.
-----------------
[osd]
         osd journal size = 10000
         osd journal = /dev/shm/journal/$name-journal
         journal dio = false
         filestore xattr use omap = true

         # The maximum interval in seconds for synchronizing the filestore.
         filestore min sync interval = 60
         filestore max sync interval = 300
-------------

Xing