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On 3/20/19 11:54 AM, Gregory Farnum
wrote:
On Tue, Mar 19, 2019 at 2:13 PM Erwin Bogaard < erwin.bogaard@xxxxxxxxx>
wrote:
Hi,
For a number of
application we use, there is a lot of file
duplication. This wastes precious storage space,
which I would like to avoid.
When using a
local disk, I can use a hard link to let all
duplicate files point to the same inode (use
“rdfind”, for example).
As there isn’t
any deduplication in Ceph(FS) I’m wondering if I can
use hard links on CephFS in the same way as I use
for ‘regular’ file systems like ext4 and xfs.
1. Is it
advisible to use hard links on CephFS? (It isn’t in
the ‘best practices’: http://docs.ceph.com/docs/master/cephfs/app-best-practices/)
This should be okay now. Hard links have changed a few
times so Zheng can correct me if I've gotten something
wrong, but the differences between regular files from a
user/performance perspective are:
* if you take snapshots and have hard links, hard-linked
files are special and will be a member of *every* snapshot
in the system (which only matters if you actually write to
them during all those snapshots)
* opening a hard-linked file may behave as if you were
doing two file opens instead of one, from a performance
perspective. But this might have changed? (In the past, you
would need to look up the file name you open, and then do
another lookup on the authoritative location of the file.)
This hasn't changed. hard link in cephfs is magic symbol link.
Its main overhead is at open.
Regards
Yan, Zheng
2. Is there any
performance (dis)advantage?
Generally not once the file is open.
3. When using
hard links, is there an actual space savings, or is
there some trickery happening?
If you create a hard link, there is a single copy of the
file data in RADOS that all the file names refer to. I think
that's what you're asking?
4. Are there any
issues (other than the regular hard link ‘gotcha’s’)
I need to keep in mind combining hard links with
CephFS?
Not other than above.
-Greg
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