Preclear and High IO Wait


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Hi,

I'm seeing very high IO wait times on my unraid server.  It seems to happen randomly, and I can't really track it down to a single thing.  The samba process on Unraid starts becoming very unresponsive.  You cannot  "open" a folder in a Windows client, it will just sit as "unresponsive" for 30 second to 1 1/2 minutes.  It does seem to happen more often when the unraid server has more files  being copied in or out.  It always happens when running preclear.  Below is a snapshot from top, the unraid server is running preclear, but this happens even when I'm not running preclear.

 

top - 10:51:14 up 1 day, 19:12,  5 users,  load average: 6.94, 6.27, 6.01

Tasks: 161 total,  1 running, 160 sleeping,  0 stopped,  0 zombie

Cpu(s):  7.4%us,  6.9%sy,  0.0%ni, 47.2%id, 37.9%wa,  0.0%hi,  0.6%si,  0.0%st

Mem:  6226524k total,  6102460k used,  124064k free,  460520k buffers

Swap:        0k total,        0k used,        0k free,  5349732k cached

 

  PID USER      PR  NI  VIRT  RES  SHR S %CPU %MEM    TIME+  COMMAND

21132 root      20  0  2152  584  436 S  34  0.0  0:01.08 sum

21131 root      20  0 10508 8876  620 D  19  0.1  0:00.59 dd

3313 root      20  0 57900 9876  592 S    5  0.2 179:30.76 shfs

14332 xbox      20  0 20036 6968 5884 S    2  0.1  1:32.47 smbd

3238 root      20  0    0    0    0 S    1  0.0  49:45.87 unraidd

20735 root      20  0  3116 1284  744 S    1  0.0  0:00.07 rsync

20737 root      20  0  2972  616  228 S    1  0.0  0:00.06 rsync

21105 odenbach  20  0 20044 4632 3624 S    1  0.1  0:00.03 smbd

21125 xbox      20  0 20044 3760 2960 S    1  0.1  0:00.03 smbd

  479 root      20  0    0    0    0 S    0  0.0  64:30.91 kswapd0

21124 root      20  0  2472 1020  756 R    0  0.0  0:00.03 top

    1 root      20  0  828  284  240 S    0  0.0  0:07.00 init

    2 root      20  0    0    0    0 S    0  0.0  0:00.06 kthreadd

    3 root      20  0    0    0    0 S    0  0.0  1:24.87 ksoftirqd/0

    5 root      0 -20    0    0    0 S    0  0.0  0:00.00 kworker/0:0H

    7 root      0 -20    0    0    0 S    0  0.0  0:00.00 kworker/u:0H

    8 root      RT  0    0    0    0 S    0  0.0  0:01.48 migration/0

 

Its a fairly powerful box.  I'm running unraid under esxi, I have dual hex core xeons, and 64GB of ram in the server.  For the unraid VM i've given it 4 cores and 6GB ram.  Running unraid 5.0.3 and passing through a M1015 controller in IT mode.  THe high load is caused by IO wait, but I don't see anything specifically.  Even if under high IO wait, I would still expect that samba would respond in a timely fashion, like in a few seconds, as opposed to several minutes.  Right now when running preclear, my samba shares are completely unusable.  I cant access them with a windows client.  When not running preclear, it happens randomly at times.  I probably have 70-100 open files at any given time on the server.  Right now, I can't even do an "hdparm -tT /dev/xxx".  It just sits and waits.  When not in the degraded condition, I can copy files out of the server at wire speed, normally around 110 Megabytes per second or so.  The degraded condition tends to come and go, but always happens when running preclear.  I would expect samba to slow down when running preclear, but I don't think it should go completely unresponsive for minutes at a time.

 

Any thoughts on this?

 

Thanks.

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  • 1 year later...

Hello.  Did anyone ever respond to your problem?  I have disabled all my dockers and there is NO IO going through the box but htop shows load averages of 4..41, 4.80, and 5.25.  The gui is ridiculously sluggish as well.  It seems as if the "dd" that is doing the preclear is killing the box.

 

I am on day 14 of a trial (v6.3.5) and am doing a preclear of a 5TB drive.  My install is a desktop i5-7500 with 16GB of RAM.  I have a 250 SSD for cache, a 6TB for parity, and a mix of WD and Seagate drives. 

 

Did you ever figure out what the problem was?

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