[gpfsug-discuss] mmapplypolicy didn't migrate everything it should have - why not?
Buterbaugh, Kevin L
Kevin.Buterbaugh at Vanderbilt.Edu
Wed Apr 19 14:12:16 BST 2017
Hi All,
I think we *may* be able to wrap this saga up… ;-)
Dave - in regards to your question, all I know is that the tail end of the log file is “normal” for all the successful pool migrations I’ve done in the past few years.
It looks like the hard links were the problem. We have one group with a fileset on our filesystem that they use for backing up Linux boxes in their lab. That one fileset has thousands and thousands (I haven’t counted, but based on the output of that Perl script I wrote it could well be millions) of files with anywhere from 50 to 128 hard links each … those files ranged from a few KB to a few MB in size.
From what Marc said, my understanding is that with the way I had my policy rule written mmapplypolicy was seeing each of those as separate files and therefore thinking it was moving 50 to 128 times as much space to the gpfs23capacity pool as it really was for those files. Marc can correct me or clarify further if necessary. He directed me to add:
SIZE(KB_ALLOCATED/NLINK)
to both of my migrate rules in my policy file. I did so and kicked off another mmapplypolicy last night, which is still running. However, the prediction section now says:
[I] GPFS Policy Decisions and File Choice Totals:
Chose to migrate 40050141920KB: 2051495 of 2051495 candidates;
Predicted Data Pool Utilization in KB and %:
Pool_Name KB_Occupied KB_Total Percent_Occupied
gpfs23capacity 104098980256 124983549952 83.290145220%
gpfs23data 168478368352 343753326592 49.011414674%
system 0 0 0.000000000% (no user data)
So now it’s going to move every file it can that matches my policies because it’s figured out that a lot of those are hard links … and I don’t have enough files matching the criteria to fill the gpfs23capacity pool to the 98% limit like mmapplypolicy thought I did before. According to the log file, it’s happily chugging along migrating files, and mmdf agrees that my gpfs23capacity pool is gradually getting more full (I have it QOSed, of course):
Disks in storage pool: gpfs23capacity (Maximum disk size allowed is 519 TB)
eon35Ansd 58.2T 35 No Yes 25.33T ( 44%) 68.13G ( 0%)
eon35Dnsd 58.2T 35 No Yes 25.33T ( 44%) 68.49G ( 0%)
------------- -------------------- -------------------
(pool total) 116.4T 50.66T ( 44%) 136.6G ( 0%)
My sincere thanks to all who took the time to respond to my questions. Of course, that goes double for Marc.
We (Vanderbilt) seem to have a long tradition of finding some edge cases in GPFS going all the way back to when we originally moved off of an NFS server to GPFS (2.2, 2.3?) back in 2005. I was creating individual tarballs of each users’ home directory on the NFS server, copying the tarball to one of the NSD servers, and untarring it there (don’t remember why we weren’t rsync’ing, but there was a reason). Everything was working just fine except for one user. Every time I tried to untar her home directory on GPFS it barfed part of the way thru … turns out that until then IBM hadn’t considered that someone would want to put 6 million files in one directory. Gotta love those users! ;-)
Kevin
On Apr 18, 2017, at 10:31 AM, David D. Johnson <david_johnson at brown.edu<mailto:david_johnson at brown.edu>> wrote:
I have an observation, which may merely serve to show my ignorance:
Is it significant that the words "EXTERNAL EXEC/script” are seen below?
If migrating between storage pools within the cluster, I would expect the PIT engine to do the migration.
When doing HSM (off cluster, tape libraries, etc) is where I would expect to need a script to actually do the work.
[I] 2017-04-18 at 09:06:51.124 Policy execution. 1620263 files dispatched.
[I] A total of 1620263 files have been migrated, deleted or processed by an EXTERNAL EXEC/script;
0 'skipped' files and/or errors.
— ddj
Dave Johnson
Brown University
On Apr 18, 2017, at 11:11 AM, Marc A Kaplan <makaplan at us.ibm.com<mailto:makaplan at us.ibm.com>> wrote:
ANYONE else reading this saga? Who uses mmapplypolicy to migrate files within multi-TB file systems? Problems? Or all working as expected?
------
Well, again mmapplypolicy "thinks" it has "chosen" 1.6 million files whose total size is 61 Terabytes and migrating those will bring the occupancy of gpfs23capacity pool to 98% and then we're done.
So now I'm wondering where this is going wrong. Is there some bug in the reckoning inside of mmapplypolicy or somewhere else in GPFS?
Sure you can put in an PMR, and probably should. I'm guessing whoever picks up the PMR will end up calling or emailing me ... but maybe she can do some of the clerical work for us...
While we're waiting for that... Here's what I suggest next.
Add a clause ...
SHOW(varchar(KB_ALLOCATED) || ' n=' || varchar(NLINK))
before the WHERE clause to each of your rules.
Re-run the command with options '-I test -L 2' and collect the output.
We're not actually going to move any data, but we're going to look at the files and file sizes that are "chosen"...
You should see 1.6 million lines that look kind of like this:
/yy/dat/bigC RULE 'msx' MIGRATE FROM POOL 'system' TO POOL 'xtra' WEIGHT(inf) SHOW( 1024 n=1)
Run a script over the output to add up all the SHOW() values in the lines that contain TO POOL 'gpfs23capacity' and verify that they do indeed
add up to 61TB... (The show is in KB so the SHOW numbers should add up to 61 billion).
That sanity checks the policy arithmetic. Let's assume that's okay.
Then the next question is whether the individual numbers are correct... Zach Giles made a suggestion... which I'll interpret as
find some of the biggest of those files and check that they really are that big....
At this point, I really don't know, but I'm guessing there's some discrepances in the reported KB_ALLOCATED numbers for many of the files...
and/or they are "illplaced" - the data blocks aren't all in the pool FROM POOL ...
HMMMM.... I just thought about this some more and added the NLINK statistic. It would be unusual for this to be a big problem, but files that are hard linked are
not recognized by mmapplypolicy as sharing storage...
This has not come to my attention as a significant problem -- does the file system in question have significant GBs of hard linked files?
The truth is that you're the first customer/user/admin in a long time to question/examine how mmapplypolicy does its space reckoning ...
Optimistically that means it works fine for most customers...
So sorry, something unusual about your installation or usage...
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