Mel wrote:
On Tuesday 16 December 2008 15:18:19 Bernard Dugas wrote:
I want to extract the list of files changed between 2 snapshots, to be
able to do efficient backups.
Just use dump(8)?
Yes in first step, may be. But for fast replication of changes ?
Imagine i have a 1TB drive with 1 partition on serverA, and i want it
duplicated on another 1TB drive serverB, with 1Gbps network between
them, so network speed is not a problem.
I begin with dump or any copy mean to have the 2 drives having the
partitions in the same state, from a first "snapshot image 0" which is
the reference on serverA, made with mksnap_ffs.
A user change only 1 small file on serverA.
After 1mn, i create "snapshot0+1" file on serverA.
To find the files changed on serverA between snapshot0 and snapshot0+1,
any current dump or equivalent will have to read all file records of
1TB partition, inside snapshot0+1 complete view of partition, to find 1
changed file. Even on nice day, it may take more than 1mn.
But the beauty and efficiency of snapshot concept is that snapshot0 file
(internal structure) contains a reference to all data blocks changed
between snapshot0 and snapshot0+1 : this is why a snapshot file is so
small and efficient.
It is certainly possible to find in snapshot0 file inside structure the
list of blocks changed until snapshot0+1 was made, and then find which
file is the owner of the changed blocks.
Then it will appear that only 1 file has been changed on whole partition
and we just have to send this changed file from snapshot0+1 view to
serverB.
So using the internal structure of snapshots file is far more efficient
than reading all directory records for last change time...
Please note that i try to use the expressions "snapshot file" and
"snapshot view" very cautiosly, because they don't mean the same at all :-)
And what snapshots do you mean? As in mksnap_ffs? Cause that's described
in /usr/src/sys/ufs/ffs/ffs_snapshot.c:
127 TAILQ_HEAD(snaphead, inode);
128
129 struct snapdata {
130 struct snaphead sn_head;
131 daddr_t sn_listsize;
132 daddr_t *sn_blklist;
133 struct lock sn_lock;
134 };
135
and not exposed to userland.
Thanks, this is a good hint ! Just needing some doc and help to
understand how to use it : i'm more in design now, programmation skills
are far away :-(
Best regards,
--
Bernard DUGAS Mobile +33 615 333 770
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