Re: [ovirt-users] xfs fragmentation problem caused data domain to hang

[Jason Keltz]

On 10/02/2017 11:05 AM, Jason Keltz wrote:

On 10/02/2017 11:00 AM, Yaniv Kaul wrote:



On Mon, Oct 2, 2017 at 5:57 PM, Jason Keltz > wrote:



On 10/02/2017 10:51 AM, Yaniv Kaul wrote:



On Mon, Oct 2, 2017 at 5:14 PM, Jason Keltz > wrote:


On 10/02/2017 01:22 AM, Yaniv Kaul wrote:



On Mon, Oct 2, 2017 at 5:11 AM, Jason Keltz
> wrote:

Hi.

For my data domain, I have one NFS server with a large
RAID filesystem (9 TB).
I'm only using 2 TB of that at the moment. Today, my
NFS server  hung with
the following error:

xfs: possible memory allocation deadlock in kmem_alloc


Can you share more of the log so we'll see what happened
before and after?
Y.


Here is engine-log from yesterday.. the problem started
around 14:29 PM.
http://www.eecs.yorku.ca/~jas/ovirt-debug/10012017/engine-log.txt



Here is the vdsm log on one of the virtualization
hosts, virt01:
http://www.eecs.yorku.ca/~jas/ovirt-debug/10012017/vdsm.log.2


Doing further investigation, I found that the XFS error
messages didn't start yesterday.  You'll see they
started at the very end of the day on September 23.  See:

http://www.eecs.yorku.ca/~jas/ovirt-debug/messages-20170924




Our storage guys do NOT think it's an XFS fragmentation
issue, but we'll be looking at it.


This is an interesting thread to read because the problem sounds quite 
similar:


http://oss.sgi.com/archives/xfs/2016-03/msg00447.html

In particular, quoted from that:

XFS maintains the full extent list for an active inode in memory,
As it is, yes, the memory allocation problem is with the in-core
extent tree, and we've known about it for some time. The issue is
that as memory gets fragmented, the top level indirection array
grows too large to be allocated as a contiguous chunk. When this
happens really depends on memory load, uptime and the way the extent
tree is being modified.


So in my case, I have a bunch of big XFS disk images for virtual disks.  
As the files are big with many extents, keeping all that information in 
memory at the same time may be the culprit.   Having many extents per se 
isn't the problem, but having enough memory to be able to store all the 
information simultaneously may be. Possible solutions would be to 
increase the default extent size of the volume (which I'm not sure how 
to do), defragment the disk, and hence less extents, or potentially add 
more memory to the file server.  It has 64G.



Hmmm... almost sorry to hear that because that would be easy
to "fix"...



They continued on the 24th, then on the 26th... I think
there were a few "hangs" on those times that people
were complaining about, but we didn't catch the
problem. However, the errors hit big time yesterday at
14:27 PM... see here:

http://www.eecs.yorku.ca/~jas/ovirt-debug/messages-20171001


If you want any other logs, I'm happy to provide them. 
I just don't know exactly what to provide.


Do you know if I can run the XFS defrag command live?
Rather than on a disk by disk, I'd rather just do it on
the whole filesystem.  There really aren't that many
files since it's just ovirt disk images.  However, I
don't understand the implications to running VMs.  I
wouldn't want to do anything to create more downtime.


Should be enough to copy the disks to make them less
fragmented.

Yes, but this requires downtime.. but there's plenty of
additional storage, so this would fix things well.



Live storage migration could be used.
Y.





I had upgraded the engine server + 4 virtualization hosts
from 4.1.1 to current on September 20 along with upgrading
them from CentOS 7.3 to CentOS 7.4.  virtfs, the NFS file
server, was running CentOS 7.3 and kernel
vmlinuz-3.10.0-514.16.1.el7.x86_64. Only yesterday, did I
upgrade it to CentOS 7.4 and hence kernel
vmlinuz-3.10.0-693.2.2.el7.x86_64.

I believe the problem is fully XFS related, and not ovirt at
all. Although, I must admit, ovirt didn't help either.  When
I rebooted the file server, the iso and export domains were

Re: [ovirt-users] xfs fragmentation problem caused data domain to hang

[Jason Keltz]

On 10/02/2017 11:00 AM, Yaniv Kaul wrote:



On Mon, Oct 2, 2017 at 5:57 PM, Jason Keltz > wrote:



On 10/02/2017 10:51 AM, Yaniv Kaul wrote:



On Mon, Oct 2, 2017 at 5:14 PM, Jason Keltz > wrote:


On 10/02/2017 01:22 AM, Yaniv Kaul wrote:



On Mon, Oct 2, 2017 at 5:11 AM, Jason Keltz
> wrote:

Hi.

For my data domain, I have one NFS server with a large
RAID filesystem (9 TB).
I'm only using 2 TB of that at the moment. Today, my NFS
server  hung with
the following error:

xfs: possible memory allocation deadlock in kmem_alloc


Can you share more of the log so we'll see what happened
before and after?
Y.


Here is engine-log from yesterday.. the problem started
around 14:29 PM.
http://www.eecs.yorku.ca/~jas/ovirt-debug/10012017/engine-log.txt



Here is the vdsm log on one of the virtualization hosts,
virt01:
http://www.eecs.yorku.ca/~jas/ovirt-debug/10012017/vdsm.log.2


Doing further investigation, I found that the XFS error
messages didn't start yesterday.  You'll see they
started at the very end of the day on September 23. See:

http://www.eecs.yorku.ca/~jas/ovirt-debug/messages-20170924




Our storage guys do NOT think it's an XFS fragmentation
issue, but we'll be looking at it.

Hmmm... almost sorry to hear that because that would be easy
to "fix"...



They continued on the 24th, then on the 26th... I think
there were a few "hangs" on those times that people were
complaining about, but we didn't catch the problem.
However, the errors hit big time yesterday at 14:27
PM... see here:

http://www.eecs.yorku.ca/~jas/ovirt-debug/messages-20171001


If you want any other logs, I'm happy to provide them. I
just don't know exactly what to provide.

Do you know if I can run the XFS defrag command live?
Rather than on a disk by disk, I'd rather just do it on
the whole filesystem. There really aren't that many
files since it's just ovirt disk images.  However, I
don't understand the implications to running VMs.  I
wouldn't want to do anything to create more downtime.


Should be enough to copy the disks to make them less fragmented.

Yes, but this requires downtime.. but there's plenty of
additional storage, so this would fix things well.



Live storage migration could be used.
Y.





I had upgraded the engine server + 4 virtualization hosts
from 4.1.1 to current on September 20 along with upgrading
them from CentOS 7.3 to CentOS 7.4.  virtfs, the NFS file
server, was running CentOS 7.3 and kernel
vmlinuz-3.10.0-514.16.1.el7.x86_64. Only yesterday, did I
upgrade it to CentOS 7.4 and hence kernel
vmlinuz-3.10.0-693.2.2.el7.x86_64.

I believe the problem is fully XFS related, and not ovirt at
all.   Although, I must admit, ovirt didn't help either. When
I rebooted the file server, the iso and export domains were
immediately active, but the data domain took quite a long
time.  I kept trying to activate it, and it couldn't do it. 
I couldn't make a host an SPM.  I found that the data domain

directory on the virtualization host was a "stale NFS file
handle".  I rebooted one of the virtualization hosts (virt1),
and tried to make it the SPM.  Again, it wouldn't work. 
Finally, I ended up turning everything into maintenance mode,
then activating just it, and I was able to make it the SPM. 
I was then able to bring everything up.  I would have

expected ovirt to handle the problem a little more
gracefully, and give me more information because I was
sweating thinking I had to restore all the VMs!


Stale NFS is on our todo list to handle. Quite challenging.

Thanks..



I didn't think when I chose XFS as the filesystem for my
virtualization NFS server that I would have to defragment the
filesystem manually.  This is like the old days of running
Norton SpeedDisk to defrag my 386...


We are still not convinced it's an issue - but we'll look into it
(and perhaps ask for more 

[ovirt-users] xfs fragmentation problem caused data domain to hang

[Jason Keltz]

Hi.

For my data domain, I have one NFS server with a large RAID filesystem (9 TB).
I'm only using 2 TB of that at the moment. Today, my NFS server  hung with
the following error:


xfs: possible memory allocation deadlock in kmem_alloc


All 4 virtualization hosts of course had problems since there was no
longer any storage.

In the end, it seems like the problem is related to XFS fragmentation...

I read this great blog here:

https://blog.codecentric.de/en/2017/04/xfs-possible-memory-allocation-deadlock-kmem_alloc/

In short, I tried this:

# xfs_db -r -c "frag -f" /dev/sdb1
actual 4314253, ideal 43107, fragmentation factor 99.00%

Apparently the fragmentation factor doesn't mean much, but the fact that
"actual" number of extents is considerably higher than "ideal" extents seems 
that it
may be the problem.

I saw that many of my virtual disks that are written to a lot have, of course,
a lot of extents...

For example, on our main web server disk image, there were 247,597
extents alone!  I took the web server down, and ran the XFS defrag
command on the disk...

# xfs_fsr -v 9a634692-1302-471f-a92e-c978b2b67fd0
9a634692-1302-471f-a92e-c978b2b67fd0
extents before:247597 after:429 DONE 9a634692-1302-471f-a92e-c978b2b67fd0

247,597 before and 429 after!  WOW!

Are virtual disks a problem with XFS?  Why isn't this memory allocation
deadlock issue more prevalent.  I do see this article mentioned on many
web posts.  I don't specifically see any recommendation to *not* use
XFS for the data domain though.

I was running CentOS 7.3 on the file server, but before rebooting the server,
I upgraded to the latest kernel and CentOS 7.4 in the hopes that if there
was a kernel issue, that this would solve it.

I took a few virtual systems down, and ran the defrag on the disks.  However,
with over 30 virtual systems, I don't really want to do this individually.
I was wondering if I could run xfs_fsr on all the disks LIVE?  It says in the
manual that you can run it live, but I can't see how this would be good when
a system is using that disk, and I don't want to deal with major
corruption across the board. Any thoughts?

Thanks,

Jason.
 


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