Re: mkfs.xfs fails with raid5 and smaller chunk sizes

[Brian Hemme <bmh@rincon.com>]

On 09/16/2014 11:02 PM, Dave Chinner wrote:
> On Tue, Sep 16, 2014 at 03:47:43PM -0700, Brian Hemme wrote:
>> On 09/16/2014 03:17 PM, Dave Chinner wrote:
>>> On Tue, Sep 16, 2014 at 03:03:08PM -0700, Brian Hemme wrote:
>>>> Hello all,
>>>>
>>>> I am having some odd problems with mkfs.xfs when used on a raid 5
>>>> array.  The array is built from 6 960GB SSDs all connected to SATA
>>>> ports on the MB and created with mdadm.  If I use a chunk size any
>>>> smaller then 512K mkfs.xfs just hangs forever.  It continues to use
>>>> CPU and so does the raid array but never completes.  If the system
>>>> is just left running for an extended length of time the whole OS
>>>> eventually locks up.  I have tried this on three different systems
>>>> with the same results.   I have searched all over for someone with
>>>> similar issues without success.  I am hoping I am just doing
>>>> something clearly wrong and you all can set me straight quickly.
>>>>
>>>> Some specifics:
>>>>      Arch linux with 3.14.1 kernel
>>>>      mkfs.xfs version 3.1.11
>>>>      mdadm - v3.3 - 3rd September 2013
>>>>
>>>> Commands:
>>>>> mdadm --create /dev/md0 --chunk=64K --level=5 --raid-devices=6
>>>> /dev/sd[a-f]
>>>>> mkfs.xfs /dev/md0
>>>>    ** This command fails and locks up
>>>>
>>>> I have tried specifying the arguments to mkfs.xfs with the same
>>>> results.  Building a 4 drive array seems to require a chunk size of
>>>> 1M or greater to work.  Same results if I make a partition on the
>>>> array and make the fs there.
>>> mkfs.xfs really should only take a couple of seconds to complete.
>>> Seeing as you are using SSDs, my first suspicion is that md or the
>>> SSDs are having problems with discard. Hence you should first
>>> try 'mkfs.xfs -K /dev/md0' and see if that completes quickly.
>>>
>>> Otherwise, output of 'echo w>   sysrq-trigger' from dmesg would be a
>>> good start, as would a 'perf top -G -U' snapshot (run for 30s at
>>> least a minute after mkfs.xfs starts) to tell us what is burning
>>> CPU.
>>>
>>> Cheers,
>>>
>>> Dave.
>> Thanks for the quick response!
>>
>> Adding the -K seemed to do the trick.  However, for my education,
>> why is this needed in this case?  It seems to work without it for
>> larger chunk sizes or for raid 0 instead of 5.
> Discard on RAID 5 can require parity recalculation if the discard
> sizes are small which means RMW operations. I'd say you probably
> need to ask the linux-raid list to debug whatever issue you are
> having with the RAID5 code.
>
>> It also worked on
>> our old install with a 3.1.6 kernel.
> RAID5 discard support was added in 3.1.7....
>
>> Any why would not using the -K
>> cause enough of a problem that the whole machine hangs?  Just trying
>> to understand this enough to make sure I don't run into problems
>> down the road.
> If you cause the IO subsystem to choke up, the system can hang
> because it can't clean dirty pages of memory and hence you can get
> ENOMEM situations that can hang the machine. Again, i'd first talk
> to the linux-raid folk to find out what is causing the RAID5 to be
> so slow in this case as it's really nothing to do with XFS at this
> point...
>
> Cheers,
>
> Dave.
Thanks very much Dave.  You got me everything I needed.

Brian

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