Providing an overdue update to wrap this thread up.
It turns out I wasn't seeing the forest for the trees.
Parallelizing the copy did in fact yield much larger results than the single threaded
In the end we used a home-brewed python script to parallelize the copy using cp, rather
than rsync, to copy things in batches, which took about 48 hours to copy ~35TiB from the
RBD to cephfs.
I think it could have gone a bit faster, however in an effort to keep the RBD NFS export
somewhat usable with respect to latency and iops.
So all in all, when in doubt, parallelize.
Otherwise, you're stuck with painfully slow single threaded performance.
On May 30, 2021, at 6:32 AM, Dave Hall
I'd like to add to Sebastian's comments - the problem is probably rsync.
I inherited a smaller setup than you when I assumed my current responsibilities - an XFS
file system on a RAID and exported over NFS. The backup process is based on RSnapshot,
which is based on rsync over SSH, but the target is another XFS on hardware RAID. The
file system contains a couple thousand user home directories for Computer Science
students, so wide and deep and lots of small files.
I was tormented by a daily backup process that was taking 3 days to run - copying the
entire file system in a single rsync. What I ultimately concluded is that rsync runs
exponentially slower as the size of the file tree to be copied increases. To get around
this, if you play some games with find and GNU parallel, you can break your file system
into many small sub-trees and run many rsyncs in parallel. Copying this way, you will see
From the point of view of a programmer, I think that rsync must build a representation of
the source and destination file trees in memory and then must traverse and re-traverse
them to make sure everything got copies and that nothing changed in the source tree.
I've never read the code, but I've see articles that confirm my theory.
In my case, because of inflexibility in RSnapshot I have ended up with 26 consecutive
rsyncs - a*, b*, c*, etc. and I still go about twice as fast as I would with one large
rsync. However, when I transferred this file system to a new NFS server and new storage I
was able to directly rsync each user in parallel. I filled up a 10GB pipe and copied the
whole FS in an hour.
Typing in a hurry. If my explanation is confusing, please don't hesitate to ask me
to explain better.
On Fri, May 28, 2021 at 11:12 AM Sebastian Knust <sknust(a)physik.uni-bielefeld.de
To add to this command by Weiwen:
On 28.05.21 13:03, 胡 玮文 wrote:
Have you tried just start multiple rsync process
simultaneously to transfer different directories? Distributed system like ceph often
benefits from more parallelism.
When I migrated from XFS on iSCSI (legacy system, no Ceph) to CephFS a
few months ago, I used msrsync  and was quite happy with the speed.
For your use case, I would start with -p 12 but might experiment with up
to -p 24 (as you only have 6C/12T in your CPU). With many small files,
you also might want to increase -s from the default 1000.
Note that msrsync does not work with the --delete rsync flag. As I was
syncing a live system, I ended up with this workflow:
- Initial sync with msrsync (something like ./msrsync -p 12 --progress
--stats --rsync "-aS --numeric-ids" ...)
- Second sync with msrsync (to sync changes during the first sync)
- Take old storage off-line for users / read-only
- Final rsync with --delete (i.e. rsync -aS --numeric-ids --delete ...)
- Mount cephfs at location of old storage, adjust /etc/exports with fsid
entries where necessary, turn system back on-line / read-write
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