On 3/20/19 11:54 AM, Gregory Farnum wrote:
On Tue, Mar 19, 2019 at 2:13 PM Erwin Bogaard <[email protected]
<mailto:[email protected]>> 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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