Weird jbd2 I/O load

DISCUSSION Andrei Banu 2 participants 8 messages Oct 15, 2013 – Oct 22, 2013

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Andrei Banu

author

2013-10-15 22:18 UTC #0 lore raw

Hello,

First off let me state that my level of knowledge and expertise is in no way a match for that of the people on this list. I am not even sure if what I want to ask is in any way related to my problem or it's just a side effect (or even plain irrelevant).

I am trying to identify the source of the problems I face with an mdraid-1 built with 2 Samsung 840 Pro SSDs. The filesystem is ext-4. I face many problems with this array:

- write speeds around 10MB/s and serious server overloads (loads of 20 to 100 - this is a quad core CPU) when copying larger files (100+ MBs):
root [~]# time dd if=arch.tar.gz of=test4 bs=2M oflag=sync
146+1 records in
146+1 records out
307191761 bytes (307 MB) copied, 23.6788 s, 13.0 MB/s

real    0m23.680s
user    0m0.000s
sys     0m0.932s

- asymmetrical wear on the 2 SSDs (one SSD has a wear of 6% while the other has a wear of 30%):
root [~]# smartctl --attributes /dev/sda | grep -i wear
177 Wear_Leveling_Count 0x0013 094% 094 000 Pre-fail Always - 196
root [~]# smartctl --attributes /dev/sdb | grep -i wear
177 Wear_Leveling_Count 0x0013 070% 070 000 Pre-fail Always - 1073

- very asymmetrical await, svctm and %util in iostat when copying larger files (100+ MB):
Device: rrqm/s wrqm/s r/s w/s rsec/s wsec/s avgrq-sz avgqu-sz await svctm %util

sda               0.00        1589.50    0.00   54.00     0.00 13148.00   243.48     0.60           11.17       0.46      2.50
sdb               0.00        1627.50    0.00   16.50     0.00 9524.00     577.21     144.25       1439.33  60.61   100.00
md1             0.00        0.00           0.00    0.00 0.00     0.00             0.00        0.00            0.00 0.00      0.00
md2             0.00        0.00           0.00    1602     0.00 12816.00     8.00        0.00            0.00         0.00      0.00
md0             0.00        0.00           0.00    0.00      0.00 0.00             0.00        0.00             0.00         0.00 0.00

- asymmetrical total LBA written but much lower than the above:
root [~]# smartctl --attributes /dev/sda | grep "Total_LBAs_Written"
241 Total_LBAs_Written 0x0032 099 099 000 Old_age Always - 23628284668
root [~]# smartctl --attributes /dev/sdb | grep "Total_LBAs_Written"
241 Total_LBAs_Written 0x0032 099 099 000 Old_age Always - 25437073579
(the gap seems to be getting narrower and narrower here though - it seems some event in the past caused this)

And the number one reason I am trying for help on this list:
root # iotop -o
Total DISK READ: 247.78 K/s | Total DISK WRITE: 495.56 K/s
TID PRIO USER DISK READ DISK WRITE SWAPIN IO> COMMAND
534 be/3 root 0.00 B/s 55.06 K/s 0.00 % 99.99 % [jbd2/md2-8]
....

When there are problems, jbd2 seems to do 99.9% I/O without doing any apparent significant reads or writes. It seems like jbd2 just keeps the devices busy.

What could be the reason of some of the above anomalies? Especially why is jbd2 keeping the raid members busy while not doing any reads or writes? Why the abysmal write speed?

So far I have updated the SSDs firmware, checked the alignment which seems ok (1MB boundary), checked with all 3 schedulers, the swap is on an md device (so the asymmetrical use and wear again can't be explained), I have looked for "hard resetting link" in dmesg but found nothing so I guess it's not a cable or back plane issue). What else can I check? What else can I try?

Kind regards!

Zheng Liu

2013-10-21 13:51 UTC #1 lore raw

Hi Andrei,

Could you please disable barrier for ext4 and try your 'dd' test again?

  $ sudo mount -t ext4 -o remount,barrier=0 ${DEV} ${MNT}

*WARNING: you could lost your data with barrier=0 when you get a power failure or cold reset.*

We have met a similar problem that is because some SSDs couldn't handle barrier command properly.

Regards,
- Zheng

Quoted 84 lines · Andrei Banu

Hello,
First off let me state that my level of knowledge and expertise is in no way a match for that of the people on this list. I am not even sure if what I want to ask is in any way related to my problem or it's just a side effect (or even plain irrelevant).
I am trying to identify the source of the problems I face with an mdraid-1 built with 2 Samsung 840 Pro SSDs. The filesystem is ext-4. I face many problems with this array:
- write speeds around 10MB/s and serious server overloads (loads of 20 to 100 - this is a quad core CPU) when copying larger files (100+ MBs):
root [~]# time dd if=arch.tar.gz of=test4 bs=2M oflag=sync 146+1 records in
146+1 records out
307191761 bytes (307 MB) copied, 23.6788 s, 13.0 MB/s
real    0m23.680s
user    0m0.000s
sys     0m0.932s
- asymmetrical wear on the 2 SSDs (one SSD has a wear of 6% while the other has a wear of 30%):
root [~]# smartctl --attributes /dev/sda | grep -i wear
177 Wear_Leveling_Count 0x0013 094% 094 000 Pre-fail
Always - 196
root [~]# smartctl --attributes /dev/sdb | grep -i wear
177 Wear_Leveling_Count 0x0013 070% 070 000 Pre-fail
Always - 1073
- very asymmetrical await, svctm and %util in iostat when copying larger files (100+ MB):
Device: rrqm/s wrqm/s r/s w/s rsec/s
wsec/s avgrq-sz avgqu-sz await svctm %util
sda 0.00 1589.50 0.00 54.00 0.00
13148.00 243.48 0.60 11.17 0.46 2.50
sdb 0.00 1627.50 0.00 16.50 0.00
9524.00 577.21 144.25 1439.33 60.61 100.00
md1 0.00 0.00 0.00 0.00 0.00
0.00 0.00 0.00 0.00 0.00 0.00
md2 0.00 0.00 0.00 1602 0.00
12816.00 8.00 0.00 0.00 0.00 0.00
md0 0.00 0.00 0.00 0.00 0.00
0.00 0.00 0.00 0.00 0.00 0.00
- asymmetrical total LBA written but much lower than the above:
root [~]# smartctl --attributes /dev/sda | grep "Total_LBAs_Written"
241 Total_LBAs_Written 0x0032 099 099 000 Old_age
Always - 23628284668
root [~]# smartctl --attributes /dev/sdb | grep "Total_LBAs_Written"
241 Total_LBAs_Written 0x0032 099 099 000 Old_age
Always - 25437073579
(the gap seems to be getting narrower and narrower here though - it seems some event in the past caused this)
And the number one reason I am trying for help on this list:
root # iotop -o
Total DISK READ: 247.78 K/s | Total DISK WRITE: 495.56 K/s
TID PRIO USER DISK READ DISK WRITE SWAPIN IO> COMMAND
534 be/3 root 0.00 B/s 55.06 K/s 0.00 % 99.99 % [jbd2/md2-8]
....
When there are problems, jbd2 seems to do 99.9% I/O without doing any apparent significant reads or writes. It seems like jbd2 just keeps the devices busy.
What could be the reason of some of the above anomalies? Especially why is jbd2 keeping the raid members busy while not doing any reads or writes? Why the abysmal write speed?
So far I have updated the SSDs firmware, checked the alignment which seems ok (1MB boundary), checked with all 3 schedulers, the swap is on an md device (so the asymmetrical use and wear again can't be explained), I have looked for "hard resetting link" in dmesg but found nothing so I guess it's not a cable or back plane issue). What else can I check? What else can I try?
Kind regards!
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Andrei Banu

author

2013-10-21 14:25 UTC #2 lore raw

Hi Zheng,

Thank you for your reply. We can make this test and if it doesn't help we'll re-enable the barrier but first I need to ask a few questions:

1. /dev/md2 is mounted on /. So your command should look like this?

$ mount -t ext4 -o remount,barrier=0    /dev/md2    /

In /etc/fstab I have other parameters as well: noatime,usrjquota=quota.user,jqfmt=vfsv0 Do I also add these like this:

$ mount -t ext4 -o remount,barrier=0,noatime,usrjquota=quota.user,jqfmt=vfsv0 /dev/md2     /

2. Can the command above be run on an active (very active) cPanel server?

3. How do I re-enable the barrier?

4. What is the probability of data loss in case of cold reboot?

Thanks!

Quoted 13 lines · Zheng Liu

Hi Andrei,
Could you please disable barrier for ext4 and try your 'dd' test again?
   $ sudo mount -t ext4 -o remount,barrier=0 ${DEV} ${MNT}
*WARNING: you could lost your data with barrier=0 when you get a power failure or cold reset.*
We have met a similar problem that is because some SSDs couldn't handle barrier command properly.
Regards,
- Zheng

Zheng Liu

2013-10-21 16:55 UTC #3 lore raw

Quoted 9 lines · Andrei Banu

Hi Zheng,
Thank you for your reply. We can make this test and if it doesn't help we'll re-enable
the barrier but first I need to ask a few questions:
1. /dev/md2 is mounted on /. So your command should look like this?
$ mount -t ext4 -o remount,barrier=0    /dev/md2    /

Yes.

Quoted 7 lines

In /etc/fstab I have other parameters as well:
noatime,usrjquota=quota.user,jqfmt=vfsv0
Do I also add these like this:
$ mount -t ext4 -o
remount,barrier=0,noatime,usrjquota=quota.user,jqfmt=vfsv0 /dev/md2     /

If I remember correctly, this command is OK. But frankly I don't do the test. I am not sure all these options will pass when you remount a partition.

Quoted 2 lines

2. Can the command above be run on an active (very active) cPanel server?

Sorry, I am not familiar with cPanel server. But if your previous testings run on a server under a heavy pressure. It quite impacts the result because disk bandwidth is occupied by this process.

Quoted 2 lines

3. How do I re-enable the barrier?

sudo mount -t ext4 -o remount,barrier=1 ${DEV} ${MNT}

Quoted 2 lines

4. What is the probability of data loss in case of cold reboot?

After disabling barrier, the system couldn't ensure that the dirty data has been written into the disk medium if your device couldn't support FUA command. So I just want to make sure that your SSD can not handle barrier command properly. *Please do not use 'barrier=0' in your product system*.

- Zheng

Quoted 22 lines

Thanks!
Quoted 13 lines · Zheng Liu
Hi Andrei,
Could you please disable barrier for ext4 and try your 'dd' test again?
   $ sudo mount -t ext4 -o remount,barrier=0 ${DEV} ${MNT}
*WARNING: you could lost your data with barrier=0 when you get a power failure or cold reset.*
We have met a similar problem that is because some SSDs couldn't handle barrier command properly.
Regards,
- Zheng
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Zheng Liu

2013-10-21 17:11 UTC #4 lore raw

Quoted 26 lines · Zheng Liu

Quoted 9 lines · Andrei Banu
Hi Zheng,
Thank you for your reply. We can make this test and if it doesn't help we'll re-enable
the barrier but first I need to ask a few questions:
1. /dev/md2 is mounted on /. So your command should look like this?
$ mount -t ext4 -o remount,barrier=0    /dev/md2    /
Yes.
Quoted 7 lines
In /etc/fstab I have other parameters as well:
noatime,usrjquota=quota.user,jqfmt=vfsv0
Do I also add these like this:
$ mount -t ext4 -o
remount,barrier=0,noatime,usrjquota=quota.user,jqfmt=vfsv0 /dev/md2     /
If I remember correctly, this command is OK. But frankly I don't do the test. I am not sure all these options will pass when you remount a partition.
Quoted 1 line

Quoted 5 lines

Quoted 1 line
2. Can the command above be run on an active (very active) cPanel server?
Sorry, I am not familiar with cPanel server. But if your previous testings run on a server under a heavy pressure. It quite impacts the result because disk bandwidth is occupied by this process.

Ah, sorry, I misunderstand your question. Yes, you can remount a partition on an active server. But as I said above, please do not run your testings on a server under heavy pressure because the result could be impacted.

Regards,
- Zheng

Quoted 40 lines

Quoted 3 lines
3. How do I re-enable the barrier?
sudo mount -t ext4 -o remount,barrier=1 ${DEV} ${MNT}
Quoted 2 lines
4. What is the probability of data loss in case of cold reboot?
After disabling barrier, the system couldn't ensure that the dirty data has been written into the disk medium if your device couldn't support FUA command. So I just want to make sure that your SSD can not handle barrier command properly. *Please do not use 'barrier=0' in your product system*.
- Zheng
Quoted 22 lines
Thanks!
Quoted 13 lines · Zheng Liu
Hi Andrei,
Could you please disable barrier for ext4 and try your 'dd' test again?
   $ sudo mount -t ext4 -o remount,barrier=0 ${DEV} ${MNT}
*WARNING: you could lost your data with barrier=0 when you get a power failure or cold reset.*
We have met a similar problem that is because some SSDs couldn't handle barrier command properly.
Regards,
- Zheng
Signature · 3 lines
To unsubscribe from this list: send the line "unsubscribe linux-ext4" in
the body of a message to [email protected]
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Andrei Banu

author

2013-10-21 17:42 UTC #5 lore raw

In reply to Zheng Liu

Hi,

Meantime I've created another md device (just 5GB) and I've redone the tests. I believe this is easier and less risky than remounting an used md device.

root [/home2]# mount -l | grep md3
/dev/md3 on /home2 type ext4 (rw,barrier=0)

root [/home2]# dd bs=2M count=64 if=/dev/zero of=test6 conv=fdatasync 64+0 records in
64+0 records out
134217728 bytes (134 MB) copied, 12.3287 s, 10.9 MB/s

So the speed issue is still with us I believe.

Is there some way to check the barrier is really set to 0?

Thanks a lot!

Quoted 13 lines · Zheng Liu

Hi Andrei,
Could you please disable barrier for ext4 and try your 'dd' test again?
   $ sudo mount -t ext4 -o remount,barrier=0 ${DEV} ${MNT}
*WARNING: you could lost your data with barrier=0 when you get a power failure or cold reset.*
We have met a similar problem that is because some SSDs couldn't handle barrier command properly.
Regards,
- Zheng

Zheng Liu

2013-10-22 02:55 UTC #6 lore raw

Quoted 9 lines · Andrei Banu

Hi,
Meantime I've created another md device (just 5GB) and I've redone the tests. I believe
this is easier and less risky than remounting an used md device.
root [/home2]# mount -l | grep md3
/dev/md3 on /home2 type ext4 (rw,barrier=0)

Quoted 6 lines

root [/home2]# dd bs=2M count=64 if=/dev/zero of=test6 conv=fdatasync 64+0 records in
64+0 records out
134217728 bytes (134 MB) copied, 12.3287 s, 10.9 MB/s
So the speed issue is still with us I believe.

Thanks for doing this. It seems that the problem we met are different.

Quoted 2 lines

Is there some way to check the barrier is really set to 0?

You have seen that from the output of 'mount' command barrier is 0. You can 'cat /proc/mounts' to double-check it. But it should be the same.

Regards,
- Zheng

Quoted 18 lines

Thanks a lot!
Quoted 13 lines · Zheng Liu
Hi Andrei,
Could you please disable barrier for ext4 and try your 'dd' test again?
  $ sudo mount -t ext4 -o remount,barrier=0 ${DEV} ${MNT}
*WARNING: you could lost your data with barrier=0 when you get a power failure or cold reset.*
We have met a similar problem that is because some SSDs couldn't handle barrier command properly.
Regards,
- Zheng

Andrei Banu

author

2013-10-22 07:22 UTC #7 lore raw

Hi,

Thank you for your attempt to help me! By the way...I have checked /proc/mounts and it was barrier=0 so...I guess that is that.

Unfortunately this leaves me with only one thing to test and that is to swap the physical SSDs with each other and see if the apparent problems move from sdb to sda or stay on sdb. If they move I guess it's the SSD on sdb. But if the problems stay I guess the problem lies elsewhere. I am just not sure it's safe to swap the 2 devices of an mdraid array.

Thanks again and kind regards!

Quoted 24 lines · Zheng Liu

Quoted 9 lines · Andrei Banu
Hi,
Meantime I've created another md device (just 5GB) and I've redone the tests. I believe
this is easier and less risky than remounting an used md device.
root [/home2]# mount -l | grep md3
/dev/md3 on /home2 type ext4 (rw,barrier=0)
Quoted 6 lines
root [/home2]# dd bs=2M count=64 if=/dev/zero of=test6 conv=fdatasync 64+0 records in
64+0 records out
134217728 bytes (134 MB) copied, 12.3287 s, 10.9 MB/s
So the speed issue is still with us I believe.
Thanks for doing this. It seems that the problem we met are different.
Quoted 1 line
Is there some way to check the barrier is really set to 0?
You have seen that from the output of 'mount' command barrier is 0. You can 'cat /proc/mounts' to double-check it. But it should be the same.
Regards,
- Zheng