Re: a question about mongodb backup using lvm snapshot

[Kay]

Hi Jinxuan Hou --

As I understand the question, then by default, the 'journal commit interval'  is 100 millisecond (unless you have overridden the 'journal commit interval' value or if you issue getLastError command).  As such, there may be ~100 millisecond window during which data durability is as risk.  Also, if you are using Amazon EBS, refer to the section in the manual.

Hope this answers your question.

Regards,

Kay

On Monday, October 22, 2012 10:34:10 PM UTC-4, Jinxuan Hou wrote:

Hi, i have a doubt about mongodb backup using lvm snapshot.

In the manual document(http://docs.mongodb.org/manual/administration/backups/), in the Backup With Journaling section it says that if you enable journal you can backup your database in following steps:

1.Create Snapshot

lvcreate --size 100M --snapshot --name mdb-snap01 /dev/vg0/mongodb

2.Archive Snapshots

umount /dev/vg0/mdb-snap01

dd if=/dev/vg0/mdb-snap01 | tar -czf mdb-snap01.tar.gz

3.Restore Snapshot

lvcreate --size 1G --name mdb-new vg0

tar -xzf mdb-snap01.tar.gz | dd of=/dev/vg0/mdb-new

mount /dev/vg0/mdb-new /srv/mongodb

4.Remote Backup Storage

umount /dev/vg0/mdb-snap01

dd if=/dev/vg0/mdb-snap01 | ssh user...@example.com tar -czf /opt/backup/mdb-snap01.tar.gz

lvcreate --size 1G --name mdb-new vg0

ssh user...@example.com tar -xzf /opt/backup/mdb-snap01.tar.gz | dd of=/dev/vg0/mdb-new

mount /dev/vg0/mdb-new /srv/mongodb

there  is not lock fsync operations necessary.

just as (http://www.mongodb.org/display/DOCS/Backups)said:As long as all files are snapshotted at the same point in time, you don't need to lock+fsync

but when taking lvm snapshot, everything are writen to the Journal log ,but the data is stealing in the file system buffer or the OS buffer is that ok for we to recover 

our data to a consistent point .(especially when the sln No.s of  Journal log and Filesystem snapshot are not consistent)

looking forward for your answer.

thank you very much .