[gpfsug-discuss] Mass UID migration suggestions

[Aaron Knister]

Oh, the one *huge* gotcha I thought I'd share-- we wrote a perl script 
to drive the migration and part of the perl script's process was to 
clone quotas from old uid numbers to the new number. I upset our GPFS 
cluster during a particular migration in which the user was over the 
grace period of the quota so after a certain point every chown() put the 
destination UID even further over its quota. The problem with this being 
that at this point every chown() operation would cause GPFS to do some 
cluster-wide quota accounting-related RPCs. That hurt. It's worth making 
sure there are no quotas defined for the destination UID numbers and if 
they are that the data coming from the source UID number will fit.

-Aaron

On 8/2/17 9:00 PM, Aaron Knister wrote:
> I'm a little late to the party here but I thought I'd share our recent 
> experiences.
> 
> We recently completed a mass UID number migration (half a billion 
> inodes) and developed two tools ("luke filewalker" and the 
> "mmilleniumfacl") to get the job done. Both luke filewalker and the 
> mmilleniumfacl are based heavily on the code in 
> /usr/lpp/mmfs/samples/util/tsreaddir.c and 
> /usr/lpp/mmfs/samples/util/tsinode.c.
> 
> luke filewalker targets traditional POSIX permissions whereas 
> mmilleniumfacl targets posix ACLs. Both tools traverse the filesystem in 
> parallel and both but particularly the 2nd, are extremely I/O intensive 
> on your metadata disks.
> 
> The gist of luke filewalker is to scan the inode structures using the 
> gpfs APIs and populate a mapping of inode number to gid and uid number. 
> It then walks the filesystem in parallel using the APIs, looks up the 
> inode number in an in-memory hash, and if appropriate changes ownership 
> using the chown() API.
> 
> The mmilleniumfacl doesn't have the luxury of scanning for POSIX ACLs 
> using the GPFS inode API so it walks the filesystem and reads the ACL of 
> any and every file, updating the ACL entries as appropriate.
> 
> I'm going to see if I can share the source code for both tools, although 
> I don't know if I can post it here since it modified existing IBM source 
> code. Could someone from IBM chime in here? If I were to send the code 
> to IBM could they publish it perhaps on the wiki?
> 
> -Aaron
> 
> On 6/30/17 11:20 AM, hpc-luke at uconn.edu wrote:
>> Hello,
>>
>>     We're trying to change most of our users uids, is there a clean 
>> way to
>> migrate all of one users files with say `mmapplypolicy`? We have to 
>> change the
>> owner of around 273539588 files, and my estimates for runtime are 
>> around 6 days.
>>
>>     What we've been doing is indexing all of the files and splitting 
>> them up by
>> owner which takes around an hour, and then we were locking the user 
>> out while we
>> chown their files. I made it multi threaded as it weirdly gave a 10% 
>> speedup
>> despite my expectation that multi threading access from a single node 
>> would not
>> give any speedup.
>>
>>     Generally I'm looking for advice on how to make the chowning 
>> faster. Would
>> spreading the chowning processes over multiple nodes improve 
>> performance? Should
>> I not stat the files before running lchown on them, since lchown 
>> checks the file
>> before changing it? I saw mention of inodescan(), in an old gpfsug 
>> email, which
>> speeds up disk read access, by not guaranteeing that the data is up to 
>> date. We
>> have a maintenance day coming up where all users will be locked out, 
>> so the file
>> handles(?) from GPFS's perspective will not be able to go stale. Is 
>> there a
>> function with similar constraints to inodescan that I can use to speed 
>> up this
>> process?
>>
>> Thank you for your time,
>>
>> Luke
>> Storrs-HPC
>> University of Connecticut
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>>
> 

-- 
Aaron Knister
NASA Center for Climate Simulation (Code 606.2)
Goddard Space Flight Center
(301) 286-2776