Hello,
On 10/31/2012 10:58 PM, Gandalf Corvotempesta wrote:
2012/10/31 Tren Blackburn <tren@xxxxxxxxxxxxxxx>:
Unless you're using btrfs which writes to the journal and osd fs
concurrently, if you lose the journal device (such as due to a
reboot), you've lost the osd device, requiring it to be remade and
re-added.
I don't understood.
Loosing a journal, will result in the whole OSD lost?
AFAIK, Ceph will write to journal. After this write it will return an "OK".
After that, the journal is wrote (in background) to a disk, so,
loosing a journal should result in loosing that portion of data, not
the whole osd.
Now, in case of 3 replicated nodes, ceph will write the same data at
the same time to the three journals? If yes, loosing a single
journal/osd should not result in loss of data, because the same data
are still on other 2 nodes. In this case, should be possible to use a
tmpfs as journal and using the replica as redundancy.
As far as I know, this is correct. You get a ACK (on the write) back
after it landed on ALL three journals (or/and osds in case of BTRFS in
parallel mode). So If you lose one node, you still have it in two more
nodes and they will commit it to disk. After recovering the missing
node/osd it will get the data from one of the other nodes. So you won't
lose any data.
Stefan
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