[lustre-discuss] Full OST, any way of avoiding it without hanging?

[Mohr Jr, Richard Frank (Rick Mohr)]

Jason,

The results you described sound like the correct behavior when you deactivate an OST on the MDS.  When you run “lctl —device <N> deactivate”, you are essentially telling the MDS to ignore that OST when it assigns stripes to a new file.  The OST will still be visible to all clients and the MDS, which allows the clients to keep reading files from that OST and allows you to delete files from the OST.  The only down side is that any file that already exists on that OST can still be written to.  Deactivating an OST is intended to stop the flow of new data to that OST while you work on removing some of the existing data, but it doesn’t actually make the OST read-only.  I think you can get the same effect from Lustre 2.9 (or newer) by using "lctl set_param osp.<osc_name>.max_create_count=0”.

I suspect that what you originally did was to deactivate the OST using something like "lctl conf_param <ost_name>.osc.active=0”.  This will notify all Lustre clients to deactivate the OST, which I believe causes the hangs you were seeing when any client tries to remove or stat a file on that OST.

--
Rick Mohr
Senior HPC System Administrator
National Institute for Computational Sciences
http://www.nics.tennessee.edu


> On Jan 7, 2019, at 11:56 AM, Jason Williams <jasonw at jhu.edu> wrote:
> 
> Sorry for the spam, but here are a few more interesting results:
> 
> If I create a file that stripes only on the full OST, and then disable the OST, I get the following:
> 
> 	• I can over write the file within it's original size and it takes up space on the "disabled" OST.
> 	• I can zero the file.
> 	• I can write more data to the file than it originally had, ie. the original file before disabling the OST was 1G, I can overwrite the file with > 1G with the OST disabled.
> 	• If I create a new file asking for that OST with the OST disabled, I get a different OST 
> 
> 
> # 4 and #2 are the only expected behavior.  I'm not sure what the behavior should be in the case of #1 and #3.
> 
> 
> --
> Jason Williams
> Assistant Director
> Systems and Data Center Operations.
> Maryland Advanced Research Computing Center (MARCC)
> Johns Hopkins University
> jasonw at jhu.edu
> 
> 
> From: lustre-discuss <lustre-discuss-bounces at lists.lustre.org> on behalf of Jason Williams <jasonw at jhu.edu>
> Sent: Monday, January 7, 2019 11:47:09 AM
> To: Mohr Jr, Richard Frank (Rick Mohr)
> Cc: lustre-discuss at lists.lustre.org
> Subject: Re: [lustre-discuss] Full OST, any way of avoiding it without hanging?
>  
> So I found this: http://wiki.lustre.org/Handling_Full_OSTs which is what I thought I had followed before but ran into hang issues.  I did some quick testing with this and found that:
> 
> 1. if I deactivate the OST in the MDS, no new files appear to be created on that OST (expected behavior) and no hangs.
> 2. If I first create a file on the OST with it activated, then deactivate the OST, and OVERWRITE a file what was spanned on that OST, the indexes stay the same and the file successfully overwrites (the file spanned 4 OSTs, so perhaps a little more testing with a single OST in the index is necessary) 
> 3. Deactivating the OST shows it as inactive in the MDS but UP in the Client. (not expected.)
> 4. I am able to delete a file that spans that OST with the OST deactivated, no hang.
> 
> I think the only thing here that concerns me a bit is #2.
> 
> --
> Jason Williams
> Assistant Director
> Systems and Data Center Operations.
> Maryland Advanced Research Computing Center (MARCC)
> Johns Hopkins University
> jasonw at jhu.edu
> 
> 
> From: lustre-discuss <lustre-discuss-bounces at lists.lustre.org> on behalf of Jason Williams <jasonw at jhu.edu>
> Sent: Sunday, January 6, 2019 5:22:16 PM
> To: Mohr Jr, Richard Frank (Rick Mohr)
> Cc: lustre-discuss at lists.lustre.org
> Subject: Re: [lustre-discuss] Full OST, any way of avoiding it without hanging?
>  
> Hi Rick, 
> I thought what I had done was disable it on the MDS, but perhaps I was following the wrong instructions. Do you know where the best instructions for what you are describing can be found? I would be willing to try again.
> 
> —
> Sent you tersely from my phone
> Jason Williams
> From: Mohr Jr, Richard Frank (Rick Mohr) <rmohr at utk.edu>
> Sent: Sunday, January 6, 2019 4:56 PM
> To: Jason Williams
> Cc: lustre-discuss at lists.lustre.org
> Subject: Re: [lustre-discuss] Full OST, any way of avoiding it without hanging?
>  
> 
> 
> > On Jan 5, 2019, at 9:49 PM, Jason Williams <jasonw at jhu.edu> wrote:
> > 
> > I have looked around the internet and found you can disable an OST, but when I have tried that, any writes (including deletes) to the OST hang the clients indefinitely. Does anyone know a way to make an OST basically "read-only" with the exception of deletes so we can work to clear out the OST? 
> 
> What command did you use to disable the OST?
> 
> There is a way to disable the OST on all the clients, but there is also a way to deactivate it on the MDS. The latter method should prevent the MDS from allocating any new files to the OST, but still allow clients to read and delete files on that OST.
> 
> --
> Rick Mohr
> Senior HPC System Administrator
> National Institute for Computational Sciences
> http://www.nics.tennessee.edu
> 
>