mkfs.xfs fails with raid5 and smaller chunk sizes

mdadm --create /dev/md0 --chunk=64K --level=5 --raid-devices=6 /dev/sd[a-f] mkfs.xfs /dev/md0

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:

Quoted 1 line

mdadm --create /dev/md0 --chunk=64K --level=5 --raid-devices=6

/dev/sd[a-f]

Quoted 1 line

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.

Quoted 20 lines · Brian Hemme

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:

Quoted 1 line

mdadm --create /dev/md0 --chunk=64K --level=5 --raid-devices=6

Quoted 8 lines

/dev/sd[a-f]

Quoted 1 line

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.

Quoted 41 lines · Dave Chinner

Quoted 20 lines · Brian Hemme

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:

Quoted 1 line

mdadm --create /dev/md0 --chunk=64K --level=5 --raid-devices=6

Quoted 8 lines

/dev/sd[a-f]

Quoted 1 line

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,

Quoted 1 line

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.

It also worked on
our old install with a 3.1.6 kernel.

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.

Quoted 40 lines · Brian Hemme

Quoted 39 lines · Dave Chinner

Quoted 20 lines · Brian Hemme

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:

Quoted 1 line

mdadm --create /dev/md0 --chunk=64K --level=5 --raid-devices=6

Quoted 8 lines

/dev/sd[a-f]

Quoted 1 line

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.

Quoted 8 lines

Quoted 3 lines

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.

Quoted 2 lines

It also worked on
our old install with a 3.1.6 kernel.

RAID5 discard support was added in 3.1.7....

Quoted 4 lines

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.