Hi!
On 12/22/21 18:29, Sascha Brawer wrote:
What storage options does the Wikimedia cloud have?
Can external
developers (i.e. people not employed by the Wikimedia foundation) write
to Cinder and/or Swift? Either from Toolforge or from Cloud VPS?
I've left more detailed replies inline. tl;dr: Currently Toolforge
doesn't really have any other options than NFS. Cloud VPS additionally
gives you the option to use Cinder (extra volumes you can attach to a VM
and move from a VM to another).
See below for context. (Actually, is this the right
list, or should I
ask elsewhere?)
For Wikidata QRank [
https://qrank.toolforge.org/
<https://qrank.toolforge.org/>], I run a cronjob on the toolforge
Kubernetes cluster. The cronjob mainly works on Wikidata dumps and
anonymized Wikimedia access logs, which it reads from the NFS-mounted
/public/dumps/public directory. Currently, the job produces 40
internal files with a total size of 21G; these files need to be
preserved between individual cronjob runs. (In a forthcoming version of
the cronjob, this will grow to ~200 files with a total size of ~40G).
For storing these intermediate files, Cinder might be a good solution.
However, afaik Cinder isn’t available on Toolforge. Therefore, I’m
currently storing the intermediate files in the account’s home directory
on NFS. Presumably (but not sure, but speculating because I’ve seen NFS
crumbling elsewhere) Wikimedia’s NFS server will be easily overloaded;
in any case, Wikimedia’s NFS server seems to protect itself by
throttling access. Because of the throttling, the cronjob is slow when
working with its intermediate files.
* Will Cinder be made available to Toolforge users? When?
We're interested in it, but no-one has time or interest to work on
making it a reality yet. This is tracked on Phabricator:
<https://phabricator.wikimedia.org/T275555>.
As a reminder: if anyone is interested in working on this or other parts
of the WMCS infrastructure, please talk to us!
* Or should I move from Toolforge to Cloud-VPS, so I
can store my
intermediate files on Cinder?
~40G is in the range where Cinder/Cloud VPS might indeed be a better
solution than NFS. While we don't currently have any official numbers on
what is acceptable on NFS and what's not, for context the Toolforge
project NFS cluster has currently about 8T of storage for about 3,000 tools.
* Or should I store my intermediate files in some
object storage? Swift?
Ceph? Something else?
WMCS currently doesn't offer direct access to any object storage
service. This is something we're likely to work on in the mid-term (next
6-12 months is the last estimate I've heard). This project is currently
stalled on some network design work:
<https://phabricator.wikimedia.org/T289882>.
* Is access to Cinder and Swift subject to the same
throttling as
NFS? Or will moving away from NFS increase the available I/O throughput?
No, NFS is subject to completely separate throttling and Ceph-backed
storage methods (local VM disks and Cinder volumes) have much higher
amount of bandwidth available.
The final output of the QRank system is a single file,
currently ~100M
in size but eventually growing to ~1G. When the cronjob has computed a
fresh version of its output, it deletes any old outputs from previous
runs (with the exception of the previous last two versions, which are
kept around internally for debugging). Typical users are other bots or
external pipelines who need a signal for prioritizing Wikidata entities,
not end users on the web. Users typically check for updates with HTTP
HEAD, or with conditional HTTP GET requests (using the standard
If-Modified-Since and If-None-Match headers). Currently, I’m serving the
output file with a custom-written HTTP server that runs as a web service
on Toolforge behind Toolforge’s nginx instance. My server reads its
content from the NFS-mounted home directory that’s getting populated by
the cronjob. Now, it’s not exactly a great idea to serve large data
files from NFS, but afaik it’s the only option available in the
Wikimedia cloud, at least for Toolforge users. Of course I might be wrong.
* Should I move from Toolforge to Cloud-VPS, so I can
serve my final
output files from Cinder instead of NFS?
* Or should I rather store my final output files in some object storage?
Swift? Ceph? Something else?
* Or is NFS just fine, even if the size of my data grows from 100M to 1G+?
When we offer object storage, yes, storing your files in it is a good
idea. I think you should be fine NFS for now (please don't quote me on
that). Cloud VPS is an option too if you prefer it.
The cronjob also uses ~5G of temporary files in /tmp, which it deletes
towards the end of each run. The temp files are used for external
sorting, so all access is sequenyoutial. I’m not sure where these temporary
files currently sit when running on Toolforge Kubernetes. Given their
volume, I presume that the tmpfs of the Kubernetes nodes will eventually
run out of memory and then fall back to disk, but I wouldn’t know how to
find this out. _If_ the backing store disk for tmpfs eventually ends up
being mounted on NFS, it sounds wasteful for the poor NFS
server;, especially since the files get deleted at job completion. In
that case, I’d love to save common resources by using a local disk. (It
doesn’t have to be an SSD; a spinning hard drive would be fine, given
the sequential access pattern). But I’m not sure how to set this up on
Toolforge Kubernetes, and I couldn’t find docs on wikitech. Actually,
this might be a micro-optimization, so perhaps not worth the trouble.
But then, I’d like to be nice with the precious shared resources in the
Wikimedia cloud.
Good question, I'm not sure either if tmpfs for Kubernetes containers is
on Ceph (SSDs) or on RAM. At least it's not on NFS.
Sorry that I couldn’t find the answers online. While
searching, I came
across the following pointers:
–
https://wikitech.wikimedia.org/wiki/Ceph
<https://wikitech.wikimedia.org/wiki/Ceph>: This page has a warning that
it’s probably “no longer true”. If the warning is correct, perhaps
the page could be deleted entirely? Or maybe it could link to the
current docs?
–
https://wikitech.wikimedia.org/wiki/Swift
<https://wikitech.wikimedia.org/wiki/Swift>: This sounds perfect, but
the page doesn’t mention how the files are getting populated, what the
ACLs are managed, and if Wikimedia’s Swift cluster is even accessible to
external developers.
–
https://wikitech.wikimedia.org/wiki/Media_storage
<https://wikitech.wikimedia.org/wiki/Media_storage>: This seems
current (I guess?), but the page doesn’t mention if/how external
Toolforge/Cloud-VPS users may upload objects, or if this is just for the
current users.
Those pages document the media storage systems used to store uploads for
the production MediaWiki projects (Wikipedia and friends). Those are not
accessible from WMCS and should be treated as completely separate
systems, and any future WMCS (object) storage services will not use them.
Documentation about the Ceph cluster powering Cloud VPS is on a separate
Wikitech page:
<https://wikitech.wikimedia.org/wiki/Portal:Cloud_VPS/Admin/Ceph>.
--
Taavi (User:Majavah)
volunteer Toolforge/Cloud VPS admin