Hello, all.
> If you set mdt.*.enable_remote_dir=1 then you can create directories that
> point back and forth across MDTs
I thought enable_remote_dir would be useful too, but it turns out that it has
changed. Patrick F. pointed out to me that it was gutted when LU-3537 was
landed for L2.8.0. Setting the option does nothing to change the behavior,
which defaults to the behavior formally made possible with enable_remote_dir=1.
Please take a look at LU-11429, which I filed to have the parameter removed.
The assessment may be wrong, please let us know.
Thanks,
-Cory
--
On 9/21/18, 11:28 PM, "lustre-discuss on behalf of Andreas Dilger"
wrote:
On Sep 20, 2018, at 16:38, Mohr Jr, Richard Frank (Rick Mohr)
wrote:
>
>
>> On Sep 19, 2018, at 8:09 PM, Colin Faber wrote:
>>
>> Why wouldn't you use DNE?
>
> I am considering it as an option, but there appear to be some potential
drawbacks.
>
> If I use DNE1, then I have to manually create directories on specific
MDTs. I will need to monitor MDT usage and make adjustments as necessary
(which is not the end of the world, but still involves some additional work).
This might be fine when I am creating new top-level directories for new
users/projects, but any existing directories created before we add a new MDT
will still only use MDT0. Since the bulk of our user/project directories will
be created early on, we still have the potential issue of running out of inodes
on MDT0.
Note that it is possible to create remote directories at any point in the
filesystem. If you set mdt.*.enable_remote_dir=1 then you can create
directories that point back and forth across MDTs. If you also set
mdt.*.enable_remote_dir_gid=-1 then all users can create remote directories.
> Based on that, I think DNE2 would be the better alternative, but it still
has similar limitations. The directories created initially will still be only
striped over a single MDT. When another MDT is added, I would need to
recursively adjust all the existing directories to have a stripe count of 2 (or
risk having MDT0 run out of inodes). Based on my understanding of how the
striped directories work, all the files in a striped directory are about evenly
split across all the MDTs that the directory is striped across (which doesn’t
work very well if MDT0 is mostly full and MDT1 is mostly empty). Most likely
we would want to have every directory striped across all MDTs, but there is a
note in the lustre manual explicitly mentioning that it’s not a good idea to do
this.
Yes, since remote and particularly striped directory creation has a
non-zero overhead due to distributed transactions and ongoing extra RPC counts
to access, it is better to limit remote and striped directories to ones that
need it.
We're working on automating the use of DNE remote/striped directories. In
2.12 it is possible to use "lfs mkdir -i -1" and "lfs mkdir -c N" to
automatically select one or more "good" MDT(s) (where "good" == least full
right now), or "lfs mkdir -i m,n,p,q" to select a disjoint list of MDTs.
> So that is why I was thinking that resizing the MDT might be the simplest
approach. Of course, I might be mistunderstanding something about DNE2, and
if that is the case, someone can correct me. Of if there are options I am not
considering, I would welcome those too.
Yes, if you are not pushing the limits of MDT size, then resizing the MDT
is a reasonable approach. This also avoids issues with MDT imbalance, which is
not ideal now, but we are working to improve.
Cheers, Andreas
---
Andreas Dilger
CTO Whamcloud
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