[Gluster-users] Diagnosing Intermittent Performance Problems Possibly Caused by Gremlins

Matt matt at mattlantis.com
Thu Feb 5 15:59:41 UTC 2015

Thanks Pranith, Will do. Sunday night we put some things in place seem 
to be mitigating it and thankfully haven't seen it again, but if we do 
I'll send the profile info to the list. I was able to collect some 
profile info under normal load.

We added some caching to some files we noticed had become really 
popular, and when that didn't entirely stop the problem, also stopped 
the most recently added gluster volume. It's odd that volume would have 
any impact as it was only used to archive backups and was almost never 
active, but several times we'd stop it during the month just because it 
was most recently added and the issue would go away, start it back up 
and it would come back. Since then it's been quiet.

On Thu, Feb 5, 2015 at 5:14 AM, Pranith Kumar Karampuri 
<pkarampu at redhat.com> wrote:
> On 02/03/2015 11:16 AM, Matt wrote:
>> Hello List,
>> So I've been frustraded by intermittent performance problems 
>> throughout January. The problem occurs on a two node setup running 
>> 3.4.5, 16 gigs of ram with a bunch of local disk. For sometimes an 
>> hour for sometimes weeks at a time (I have extensive graphs in 
>> OpenNMS) our Gluster boxes will get their CPUs pegged, and in vmstat 
>> they'll show extremely high numbers of context switches and 
>> interrupts. Eventually things calm down. During this time, memory 
>> usage actually drops. Overall usage on the box goes from between 
>> 6-10 gigs to right around 4 gigs, and stays there. That's what 
>> really puzzles me.
>> When performance is problematic, sar shows one device, the device 
>> corresponding to the glusterfsd problem using all the CPU doing lots 
>> of little reads, Sometimes 70k/second, very small avg rq size, say 
>> 10-12. Afraid I don't have any saved output handy, but I can try to 
>> capture some next time it happens. I have tons of information 
>> frankly, but am trying to keep this reasonably brief.
>> There are more than a dozen volumes on this two node setup. The CPU 
>> usage is pretty much entirely contained to one volume, a 1.5 TB 
>> volume that is just shy of 70% full. It stores uploaded files for a 
>> web app. What I hate about this app and so am always suspicious of, 
>> is that it stores a directory for every user in one level, so under 
>> the /data directory in the volume, there are 450,000 sub directories 
>> at this point.
>> The only real mitigation step that's been taken so far was to turn 
>> off the self-heal daemon on the volume, as I thought maybe crawling 
>> that large directory was getting expensive. This doesn't seem to 
>> have done anything as the problem still occurs.
>> At this point I figure there are one of two things sorts of things 
>> happening really broadly: one we're running into some sort of bug or 
>> performance problem with gluster we should either fix perhaps by 
>> upgrading or tuning around, or two, some process we're running but 
>> not aware of is hammering the file system causing problems.
>> If it's the latter option, can anyone give me any tips on figuring 
>> out what might be hammering the system? I can use volume top to see 
>> what a brick is doing, but I can't figure out how to tell what 
>> clients are doing what.
>> Apologies for the somewhat broad nature of the question, any input 
>> thoughts would be much appreciated. I can certainly provide more 
>> info about some things if it would help, but I've tried not to write 
>> a novel here.
>> Thanks,
> Could you enable 'gluster volume profile <volname> start' for this 
> volume?
> When next time this issue happens, keep collecting 'gluster volume 
> profile <volname> info' outputs. Mail them and lets see what is 
> happening.
> Pranith
>> -Matt
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