Arne Jansen wrote:
Hi,
I have a setup with thousands of filesystems, each containing several
snapshots. For a good percentage of these filesystems I want to create
a snapshots once every hour, for others once every 2 hours and so forth.
I built some tools to do this, no problem so far.
While examining disk load on the system, I found out that load jumps
up whenever the snapshot creation process is running. Delving a bit
deeper it seems that every snapshot start a new txg. This seems to be
quite costly, about 100-150 IOPs. This takes about one second, so I can
create one snapshot per second. Creating all necessary snapshot for one
hour takes about 45 minutes. During this time the disks are at 70%
utilization and txgs are back-to-back. So I need to optimize this.
Looking at the code it seems that recursive snapshots are being
collected into a single txg. So my aim is to collect all necessary
snapshots in a single txg, too. Using libzfs or ioctl I haven't found
any way to do this. I cannot just use recursive snapshots, because
not all filesystems need to be snapshotted. Same with snapshot deletion.
My idea is to write a small kernel module that roughly duplicates the
code of zfs_ioc_snapshot, but instead of being fully recursive gets
passed a list of filesystems to snapshot.
My questions:
- is there any easier way to bring down disk load and accelerate
snapshot creation?
- are there any arguments, why my approach isn't feasible?
Just as a followup, the module works smoothly for creations. If I take
100 snapshots at a time, the gain is roughly a factor of 100 :)
Because there are no recursive deletions in kernel space (recursive
deletions are handled by libzfs), I haven't found an easy way to do
likewise for delete. Maybe someday when I get a better grip on the
source...
--Arne
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