Re: Runaway SLAB usage by 'bio' during 'device replace'

[g6094199]

Hi Chris,


since you are using a recent LTS kernel on your centos/rockstor, i guess
the kernel errors might help to find some bugs here.

can you give the devs the errors from your logs?
additionally basic info on your raid settings would be nice to, but
which specific details the devs should ask on demand.


But generally speaking raid5/6 works quiet ok in every day use for less
important data, but there a major bugs when it comes to failing disks or
in general when you try to replace harddrives.
I have a similar problem right now. I added a new drive to an array and
while deleting an older drive the new drive failed :-( So i ended up
rescuing all data (8TB) to an new array with "btrfs restore". This took
over a week, cause there is currently no switch to automaticly cancel
looping while recovering. So you will have to manually apply the cancel
command on every file it starts to loop, which might be a lot.

In general adding a new drive and afterwards removing the old one is
more save than the replace method, at least right now (as of kernel
4.5/4.6). But major bug fixes are in the works and there is hope that
raid5/6 becomes more reliable next year.


so good luck!


Am 30.05.2016 um 22:55 schrieb Duncan:
> Chris Johnson posted on Mon, 30 May 2016 11:48:02 -0700 as excerpted:
>
>> I have a RAID6 array that had a failed HDD. The drive failed completely
>> and has been removed from the system. I'm running a 'device replace'
>> operation with a new disk. The array is ~20TB so this will take a few
>> days.
> This isn't a direct answer to your issue as I'm a user and list regular, 
> not a dev, and that's beyond me, but it's something you need to know, if 
> you don't already...
>
> Btrfs raid56 mode remains for the time being in general negatively-
> recommended, except specifically for testing with throw-away data, due to 
> two critical but not immediately data destroying bugs, one related to 
> serial device replacement, the other to balance restriping.  They may or 
> may not be related to each other, as neither one has been fully traced.
>
> The serial replace bug has to do with replacing multiple devices, one at 
> a time.  The first replace appears to work fine by all visible measures, 
> but apparently doesn't return the array to full working condition after 
> all, because an attempt to replace a second device fails, and can bring 
> down the filesystem.  Unfortunately it doesn't always happen, and due to 
> the size of devices these days, working arrays tend to be multi-TB 
> monsters that take time to get to this point, so all we have at this 
> point is multiple reports of the same issue, but no real way to reproduce 
> it.  I believe but am not sure that the problem can occur regardless of 
> whether btrfs replace or device add/delete was used.
>
> The restriping bug has to do with restriping to a different width, either 
> manually doing a filtered balance after adding devices, or automatically, 
> as triggered by btrfs device delete.  Again, multiple reports but not 
> nailed down to anything specifically reproducible yet.  The problem here 
> is that the restripes, while apparently producing correct results, can 
> inexplicably take an order of magnitude (or worse) longer than they 
> should.  What one might expect to take hours takes over a week, and on 
> the big arrays that might be expected to take 2-3 days, months.
>
> The problem, again, isn't correctness, but the fact that over such long 
> periods, the risk of device loss is increased, and if the array was 
> already being reshaped/rebalanced to repair loss of one device, loss of 
> another device may kill it.
>
> Neither of these bugs affects normal runtime operation, but both are 
> critical enough with regard to what people normally use parity-raid for, 
> so they /can/ take a device (or two with raid6) loss and repair the array 
> to get back to normal operation, that raid56 remains negatively 
> recommended for anything but testing with throw-away data, until after 
> these bugs can be fully traced and fixed.
>
>
> Your particular issue doesn't appear to be directly related to either of 
> the above.  In fact, I know I've seen patches recently having to do with 
> memory leaks that may well fix your problem (tho you'd have to be running 
> 4.6 at least to have them at this point, and perhaps even 4.7-rc1.
>
> But given the situation, either be sure you have backups and are prepared 
> to use them if the array goes south on you due to failed or impractical 
> device replacement, or switch to something other than btrfs raid56 mode.  
> Btrfs redundancy-raid (raid1 and raid10) are more mature and tested, and 
> thus may be options if they fit your filesystem space and device layout 
> needs.  Alternatively, btrfs (or other filesystems) on top of dm/md-raid 
> may be an option, tho you obviously lose some features of btrfs that 
> way.  And of course zfs is the closest btrfs-comparable that's reasonably 
> mature and may be an 

Re: Runaway SLAB usage by 'bio' during 'device replace'

[Scott Talbert]

On Tue, 31 May 2016, Filipe Manana wrote:


On Mon, May 30, 2016 at 7:48 PM, Chris Johnson  wrote:

I have a RAID6 array that had a failed HDD. The drive failed
completely and has been removed from the system. I'm running a 'device
replace' operation with a new disk. The array is ~20TB so this will
take a few days.

Yesterday the system crashed hard with OOM errors about 24 hours into
the replace. Rebooting after the crash and remounting the array
automatically resumed the replace where it left off.

Today I kept a close eye on it and have watched the memory usage creep
up slowly.

htop says this is user process memory (green bar) but shows no user
processes using this much memory

free says this is almost entirely cached/buffered memory that is
taking up the space.

slabtop reveals that there is a highly unusual amount of SLAB going to
'bio' which has to do with block allocation apparently. slabtop output
is attached.

'sync && echo 3 > /proc/sys/vm/drop_caches' clears the high usage
(~4GB) from dentry but 'bio' does not release any (11GB) memory and
continues to grow slowly.


Probably you are experiencing a leak that was recently fixed and, at
the moment, available only in the 4.7-rc1 kernel:

https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=4673272f43ae790ab9ec04e38a7542f82bb8f020


Yes, you would almost certainly be hitting that memory leak.


This is running the Rockstor distro based on CentOS. The system has 16GB of RAM.

Kernel: 4.4.5-1.el7.elrepo.x86_64
btrfs-progs: 4.4.1

Kernel messages aren't showing anything of note during the replace
until it starts throwing out OOM errors.

I would like to collect enough information for a useful bug report
here, but I also can't babysit this rebuild during the work week and
reboot it once a day for OOM crashes. Should I cancel the replace
operation and use 'dev delete missing' instead? Will using 'delete
missing' cause any problem if it's done after a partially completed
and canceled replace?


If you can't get a kernel with the memory leak patched, 'dev delete missing' 
doesn't suffer from the memory leak, so it's possible you could use that. 
Also, in our testing we've seen 'dev delete missing' to be more reliable 
than replace.


As to whether it will be problematic to cancel the replace and do a delete 
missing - that I'm not sure.


Scott
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Re: Runaway SLAB usage by 'bio' during 'device replace'

[Filipe Manana]

On Mon, May 30, 2016 at 7:48 PM, Chris Johnson  wrote:
> I have a RAID6 array that had a failed HDD. The drive failed
> completely and has been removed from the system. I'm running a 'device
> replace' operation with a new disk. The array is ~20TB so this will
> take a few days.
>
> Yesterday the system crashed hard with OOM errors about 24 hours into
> the replace. Rebooting after the crash and remounting the array
> automatically resumed the replace where it left off.
>
> Today I kept a close eye on it and have watched the memory usage creep
> up slowly.
>
> htop says this is user process memory (green bar) but shows no user
> processes using this much memory
>
> free says this is almost entirely cached/buffered memory that is
> taking up the space.
>
> slabtop reveals that there is a highly unusual amount of SLAB going to
> 'bio' which has to do with block allocation apparently. slabtop output
> is attached.
>
> 'sync && echo 3 > /proc/sys/vm/drop_caches' clears the high usage
> (~4GB) from dentry but 'bio' does not release any (11GB) memory and
> continues to grow slowly.

Probably you are experiencing a leak that was recently fixed and, at
the moment, available only in the 4.7-rc1 kernel:

https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=4673272f43ae790ab9ec04e38a7542f82bb8f020

>
> This is running the Rockstor distro based on CentOS. The system has 16GB of 
> RAM.
>
> Kernel: 4.4.5-1.el7.elrepo.x86_64
> btrfs-progs: 4.4.1
>
> Kernel messages aren't showing anything of note during the replace
> until it starts throwing out OOM errors.
>
> I would like to collect enough information for a useful bug report
> here, but I also can't babysit this rebuild during the work week and
> reboot it once a day for OOM crashes. Should I cancel the replace
> operation and use 'dev delete missing' instead? Will using 'delete
> missing' cause any problem if it's done after a partially completed
> and canceled replace?



-- 
Filipe David Manana,

"Reasonable men adapt themselves to the world.
 Unreasonable men adapt the world to themselves.
 That's why all progress depends on unreasonable men."
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Re: Runaway SLAB usage by 'bio' during 'device replace'

[Duncan]

Chris Johnson posted on Mon, 30 May 2016 11:48:02 -0700 as excerpted:

> I have a RAID6 array that had a failed HDD. The drive failed completely
> and has been removed from the system. I'm running a 'device replace'
> operation with a new disk. The array is ~20TB so this will take a few
> days.

This isn't a direct answer to your issue as I'm a user and list regular, 
not a dev, and that's beyond me, but it's something you need to know, if 
you don't already...

Btrfs raid56 mode remains for the time being in general negatively-
recommended, except specifically for testing with throw-away data, due to 
two critical but not immediately data destroying bugs, one related to 
serial device replacement, the other to balance restriping.  They may or 
may not be related to each other, as neither one has been fully traced.

The serial replace bug has to do with replacing multiple devices, one at 
a time.  The first replace appears to work fine by all visible measures, 
but apparently doesn't return the array to full working condition after 
all, because an attempt to replace a second device fails, and can bring 
down the filesystem.  Unfortunately it doesn't always happen, and due to 
the size of devices these days, working arrays tend to be multi-TB 
monsters that take time to get to this point, so all we have at this 
point is multiple reports of the same issue, but no real way to reproduce 
it.  I believe but am not sure that the problem can occur regardless of 
whether btrfs replace or device add/delete was used.

The restriping bug has to do with restriping to a different width, either 
manually doing a filtered balance after adding devices, or automatically, 
as triggered by btrfs device delete.  Again, multiple reports but not 
nailed down to anything specifically reproducible yet.  The problem here 
is that the restripes, while apparently producing correct results, can 
inexplicably take an order of magnitude (or worse) longer than they 
should.  What one might expect to take hours takes over a week, and on 
the big arrays that might be expected to take 2-3 days, months.

The problem, again, isn't correctness, but the fact that over such long 
periods, the risk of device loss is increased, and if the array was 
already being reshaped/rebalanced to repair loss of one device, loss of 
another device may kill it.

Neither of these bugs affects normal runtime operation, but both are 
critical enough with regard to what people normally use parity-raid for, 
so they /can/ take a device (or two with raid6) loss and repair the array 
to get back to normal operation, that raid56 remains negatively 
recommended for anything but testing with throw-away data, until after 
these bugs can be fully traced and fixed.


Your particular issue doesn't appear to be directly related to either of 
the above.  In fact, I know I've seen patches recently having to do with 
memory leaks that may well fix your problem (tho you'd have to be running 
4.6 at least to have them at this point, and perhaps even 4.7-rc1.

But given the situation, either be sure you have backups and are prepared 
to use them if the array goes south on you due to failed or impractical 
device replacement, or switch to something other than btrfs raid56 mode.  
Btrfs redundancy-raid (raid1 and raid10) are more mature and tested, and 
thus may be options if they fit your filesystem space and device layout 
needs.  Alternatively, btrfs (or other filesystems) on top of dm/md-raid 
may be an option, tho you obviously lose some features of btrfs that 
way.  And of course zfs is the closest btrfs-comparable that's reasonably 
mature and may be an option, tho there are licensing and hardware issues 
(it likes lots of memory on linux due to double-caching of some elements 
as its caching scheme doesn't work well with that of linux, and ecc 
memory is very strongly recommended) if using it on linux.

I'd suggest giving btrfs raid56 another few kernel releases, six months 
to a year, and then check back.  I'd hope the bugs can be properly traced 
and fixed within a couple kernel cycles, so four months or so, but I 
prefer a few cycles to stabilize with no known critical bugs, before I 
recommend it (I was getting close to recommending it after the last known 
critical bug was fixed in 4.1, when these came up), which puts the 
projected timeframe at 8-12 months, before I could really consider raid56 
mode as reasonably stable as btrfs in general, which is to say, 
stabilizing, but not yet fully stable, so even then, the standard admin 
backup rule that if you don't have backups you consider the data to be 
worth less than the time/resources/hassle to do those backups, still 
applies more strongly than it would to a fully mature filesystem.

-- 
Duncan - List replies preferred.   No HTML msgs.
"Every nonfree program has a lord, a master --
and if you use the program, he is your master."  Richard Stallman

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Runaway SLAB usage by 'bio' during 'device replace'

[Chris Johnson]

I have a RAID6 array that had a failed HDD. The drive failed
completely and has been removed from the system. I'm running a 'device
replace' operation with a new disk. The array is ~20TB so this will
take a few days.

Yesterday the system crashed hard with OOM errors about 24 hours into
the replace. Rebooting after the crash and remounting the array
automatically resumed the replace where it left off.

Today I kept a close eye on it and have watched the memory usage creep
up slowly.

htop says this is user process memory (green bar) but shows no user
processes using this much memory

free says this is almost entirely cached/buffered memory that is
taking up the space.

slabtop reveals that there is a highly unusual amount of SLAB going to
'bio' which has to do with block allocation apparently. slabtop output
is attached.

'sync && echo 3 > /proc/sys/vm/drop_caches' clears the high usage
(~4GB) from dentry but 'bio' does not release any (11GB) memory and
continues to grow slowly.

This is running the Rockstor distro based on CentOS. The system has 16GB of RAM.

Kernel: 4.4.5-1.el7.elrepo.x86_64
btrfs-progs: 4.4.1

Kernel messages aren't showing anything of note during the replace
until it starts throwing out OOM errors.

I would like to collect enough information for a useful bug report
here, but I also can't babysit this rebuild during the work week and
reboot it once a day for OOM crashes. Should I cancel the replace
operation and use 'dev delete missing' instead? Will using 'delete
missing' cause any problem if it's done after a partially completed
and canceled replace?
# slabtop -o -s=a
 Active / Total Objects (% used): 33431432 / 33664160 (99.3%)
 Active / Total Slabs (% used)  : 1346736 / 1346736 (100.0%)
 Active / Total Caches (% used) : 78 / 114 (68.4%)
 Active / Total Size (% used)   : 10512136.19K / 10737701.80K (97.9%)
 Minimum / Average / Maximum Object : 0.01K / 0.32K / 15.62K

  OBJS ACTIVE  USE OBJ SIZE  SLABS OBJ/SLAB CACHE SIZE NAME   
32493650 32492775  99%0.31K 1299746   25  10397968K bio-1   
   
323505 323447  99%0.19K  15405   21 61620K dentry 
176680 176680 100%0.07K   3155   56 12620K btrfs_free_space   
118208  41288  34%0.12K   3694   32 14776K kmalloc-128
 94528  43378  45%0.25K   2954   32 23632K kmalloc-256
 91872  41682  45%0.50K   2871   32 45936K kmalloc-512
 83048  39031  46%4.00K  103818332192K kmalloc-4096   
 69049  69049 100%0.27K   2381   29 19048K btrfs_extent_buffer
 46872  46385  98%0.57K   1674   28 26784K radix_tree_node
 23460  23460 100%0.12K690   34  2760K kernfs_node_cache  
 17536  17536 100%0.98K548   32 17536K btrfs_inode
 16380  16007  97%0.14K585   28  2340K btrfs_path 
 12444  11635  93%0.08K244   51   976K Acpi-State 
 12404  12404 100%0.55K443   28  7088K inode_cache
 11648  10851  93%0.06K182   64   728K kmalloc-64 
 10404   5716  54%0.08K204   51   816K btrfs_extent_state 
  8954   8703  97%0.18K407   22  1628K vm_area_struct 
  5888   4946  84%0.03K 46  128   184K kmalloc-32 
  5632   5632 100%0.01K 11  51244K kmalloc-8  
  5049   4905  97%0.08K 99   51   396K anon_vma   
  4352   4352 100%0.02K 17  25668K kmalloc-16 
  3723   3723 100%0.05K 51   73   204K Acpi-Parse 
  3230   3230 100%0.05K 38   85   152K ftrace_event_field 
  3213   2949  91%0.19K153   21   612K kmalloc-192
  3120   3090  99%0.61K120   26  1920K proc_inode_cache   
  2814   2814 100%0.09K 67   42   268K kmalloc-96 
  1984   1510  76%1.00K 62   32  1984K kmalloc-1024   
  1904   1904 100%0.07K 34   56   136K Acpi-Operand   
  1472   1472 100%0.09K 32   46   128K trace_event_file   
  1224   1224 100%0.04K 12  10248K Acpi-Namespace 
  1152   1152 100%0.64K 48   24   768K shmem_inode_cache  
   592581  98%2.00K 37   16  1184K kmalloc-2048   
   528457  86%0.36K 24   22   192K blkdev_requests
   462355  76%0.38K 22   21   176K mnt_cache  
   450433  96%1.06K 15   30   480K signal_cache   
   429429 100%0.20K 11   3988K btrfs_delayed_ref_head 
   420420 100%2.05K 28   15   896K idr_layer_cache
   408408 100%0.04K  4  102