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I am bringing to ServerFault a problem that is tormenting me for 6+ months. I have a CentOS 6 (64bit) server with an md software raid-1 array with 2 x Samsung 840 Pro SSDs (512GB).

Problems:

  • Serious write speed problems:
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
  • When doing the above (or any other larger copy) the load spikes to unbelievable values (even over 100) going up from ~ 1.

  • When doing the above I've also noticed very weird iostat results:

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 1602.00     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

And it keeps it this way until it actually writes the file to the device (out from swap/cache/memory).

The problem is that the second SSD in the array has svctm and await roughly 100 times larger than the second.

  • For some reason the wear is different between the 2 members of the array
root [~]# smartctl --attributes /dev/sda | grep -i wear
177 Wear_Leveling_Count     0x0013   094%   094   000    Pre-fail  Always       -       180
root [~]# smartctl --attributes /dev/sdb | grep -i wear
177 Wear_Leveling_Count     0x0013   070%   070   000    Pre-fail  Always       -       1005

The first SSD has a wear of 6% while the second SSD has a wear of 30%!!

It's like the second SSD in the array works at least 5 times as hard as the first one as proven by the first iteration of iostat (the averages since reboot):

Device:         rrqm/s   wrqm/s     r/s     w/s   rsec/s   wsec/s avgrq-sz avgqu-sz   await  svctm  %util
sda              10.44    51.06  790.39  125.41  8803.98  1633.11    11.40     0.33    0.37   0.06   5.64
sdb               9.53    58.35  322.37  118.11  4835.59  1633.11    14.69     0.33    0.76   0.29  12.97
md1               0.00     0.00    1.88    1.33    15.07    10.68     8.00     0.00    0.00   0.00   0.00
md2               0.00     0.00 1109.02  173.12 10881.59  1620.39     9.75     0.00    0.00   0.00   0.00
md0               0.00     0.00    0.41    0.01     3.10     0.02     7.42     0.00    0.00   0.00   0.00
  • What I've tried: I've updated the firmware to DXM05B0Q (following reports of dramatic improvements for 840Ps after this update). I have looked for "hard resetting link" in dmesg to check for cable/backplane issues but nothing. I have checked the alignment and I believe they are aligned correctly (1MB boundary, listing below) I have checked /proc/mdstat and the array is Optimal (second listing below).
root [~]# fdisk -ul /dev/sda
Disk /dev/sda: 512.1 GB, 512110190592 bytes
255 heads, 63 sectors/track, 62260 cylinders, total 1000215216 sectors
Units = sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0x00026d59

Device Boot Start End Blocks Id System
/dev/sda1 2048 4196351 2097152 fd Linux raid autodetect
Partition 1 does not end on cylinder boundary.
/dev/sda2 * 4196352 4605951 204800 fd Linux raid autodetect
Partition 2 does not end on cylinder boundary.
/dev/sda3 4605952 814106623 404750336 fd Linux raid autodetect


root [~]# fdisk -ul /dev/sdb
Disk /dev/sdb: 512.1 GB, 512110190592 bytes
255 heads, 63 sectors/track, 62260 cylinders, total 1000215216 sectors
Units = sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0x0003dede

Device Boot Start End Blocks Id System
/dev/sdb1 2048 4196351 2097152 fd Linux raid autodetect
Partition 1 does not end on cylinder boundary.
/dev/sdb2 * 4196352 4605951 204800 fd Linux raid autodetect
Partition 2 does not end on cylinder boundary.
/dev/sdb3 4605952 814106623 404750336 fd Linux raid autodetect
/proc/mdstat
root # cat /proc/mdstat
Personalities : [raid1]
md0 : active raid1 sdb2[1] sda2[0]
204736 blocks super 1.0 [2/2] [UU]

md2 : active raid1 sdb3[1] sda3[0]
404750144 blocks super 1.0 [2/2] [UU]

md1 : active raid1 sdb1[1] sda1[0]
2096064 blocks super 1.1 [2/2] [UU]

unused devices: 
  • Running a read test with hdparm
root [~]# hdparm -t /dev/sda
/dev/sda:
 Timing buffered disk reads:  664 MB in  3.00 seconds = 221.33 MB/sec

root [~]# hdparm -t /dev/sdb
/dev/sdb:
 Timing buffered disk reads:  288 MB in  3.01 seconds =  95.77 MB/sec
  • But look what happens if I add --direct
root [~]# hdparm --direct -t /dev/sda

/dev/sda:
 Timing O_DIRECT disk reads:  788 MB in  3.01 seconds = 262.08 MB/sec
root [~]# hdparm --direct -t /dev/sdb

/dev/sdb:
 Timing O_DIRECT disk reads:  534 MB in  3.02 seconds = 176.90 MB/sec

Both tests increase but /dev/sdb doubles while /dev/sda increases maybe 20%. I just don't know what to make of this.

  • As suggested by Mr. Wagner I've done another read test with dd this time and it confirms the hdparm test:
root [/home2]# dd if=/dev/sda of=/dev/null bs=1G count=10
10+0 records in
10+0 records out
10737418240 bytes (11 GB) copied, 38.0855 s, 282 MB/s

root [/home2]# dd if=/dev/sdb of=/dev/null bs=1G count=10
10+0 records in
10+0 records out
10737418240 bytes (11 GB) copied, 115.24 s, 93.2 MB/s

So sda is 3 times faster than sdb. Or maybe sdb is doing also something else besides what sda does. Is there some way to find out if sdb is doing more than what sda does?

  • UPDATE

Again, as suggested by Mr. Wagner, I have swapped the 2 SSDs. And as he thought it would happen, the problem moved from sdb to sda. So I guess I'll RMA one of the SSDs. I wonder if the cage might be problematic.

What is wrong with this array? Please help!

share|improve this question
    
Even though there are no issues reported I suspect the hardware. If you swap the drives what do you see? Either the poor performance stays with sdb (indicating a port / cabling issue) or the poor performance moves to sda (indicating a drive issue). You could also try running hdparm -t /dev/sda (and for b) or dd if=/dev/sda of=/dev/null bs=1G count=10 (and for b) to get more data. –  Mark Wagner Oct 18 '13 at 23:58
    
@MarkWagner 1. Is it absolutely safe to swap the 2 members of the same MD raid1 array on an active server (with plenty of important data on it)? 2. I've added 2 kinds of hdparm tests above (they do look interesting - forgot to mention those in my initial post). 3. The dd command you've mentioned is safe on an active server with plenty of important data on it? –  Andy B Oct 19 '13 at 5:50
    
1. Depending on how booting is configured, swapping the drives may result in an unbootable system. 3. The dd command is non-destructive. Is it safe? It is hard to say how the system will respond when you have failing hardware. –  Mark Wagner Oct 21 '13 at 17:39
    
@MarkWagner I've done the dd read test and it confirms the hdparm test (added the listing above). I have not yet done the devices swapping test...still hesitating. Booting is setup in both devices and in the worse case we can swap them again (so no results but at least we can get the system to boot). But I still fear some data loss. –  Andy B Oct 21 '13 at 18:28
    
@MarkWagner In the end, I took the chance and swapped the 2 devices. As you theorized, all the problems (read speed issues, iostat revealed issues) moved along. I'll ask for a new SSD, perform the resync and update this thread. Meanwhile thank you! –  Andy B Oct 22 '13 at 16:13

1 Answer 1

up vote 0 down vote accepted

Well in the end I guess I found out at least a big part of the problem: one of the SSDs in the array was functioning really badly. I have read enough reports of poor mdraid performance concerning Samsung 840 Pro SSDs but this drive was functioning very poorly even when used by itself. For the time being I have fixed it by performing a secure erase of the SSD in question by using hdparm. The performance is nothing to brag about but it's much closer to decent than before: about 210-220MB/s read and about 130-150MB/s write (compared to 5-10MB/s write before). Please note this is on SATA2 where the top speed is about 240MB/s.

In the end I'd like to say thanks to Mr. Wagner who's advice to swap the drives proved illuminating.

In conclusion when you have performance issues with a SSD consider a secure erase! Please note a secure erase is not the same with formating.

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