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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