I'm using Ubuntu 15.04 with MD RAID1, LVM and LXC on top of them as a webserver. I have normal load average about 1-2, and all is working well until I perfrom any massive IO operation. The strange thing is that I'm doing these operations on unused disks. So, for example, sdc and sdd are in RAID1 used by webserver and I'm doing dd if=/dev/zero of=/dev/sdb, or just simply running du. After few seconds, load average jumps to 10 and more and the system became almost unusable. Nmon shows that all disks became overloaded, and if I would not stop the operation, overloading will continue to grow.

I'm attaching screenshots of nmon. [Overload with dd if=/dev/zero of=/dev/sdb](https://i.stack.imgur.com/PPvDP.png).

Output of uname:

> 
> ... 3.19.0-39-generic #44-Ubuntu SMP Tue Dec 1 14:39:05 UTC 2015
>  x86\_64 x86\_64 x86\_64 GNU/Linux
> 
> 
> 

**UPDATE:**

I have changed disk configuration, replaced system disks with more robust HGST. System disk aliases have been changed after the system rebooting.

Output of lsblk:

```
NAME                     FSTYPE              SIZE MOUNTPOINT
sda                                          1,8T 
├─sda1                                         1M 
├─sda2                   linux_raid_member    50G 
│ └─md0                  LVM2_member          50G 
│   ├─system-swap        swap                 10G [SWAP]
│   └─system-root        ext4                 40G /
└─sda3                   linux_raid_member 881,5G 
  └─md1                  LVM2_member       881,4G 
    ├─lxc-hosting        ext3                450G 
    ├─lxc-ns1-real                             1G 
    │ ├─lxc-ns1          ext3                  1G 
    │ └─lxc-ns1--snap6   ext3                  1G 
    └─lxc-ns1--snap6-cow                       1G 
      └─lxc-ns1--snap6   ext3                  1G 
sdb                                          1,8T 
├─sdb1                                         1M 
├─sdb2                   linux_raid_member    50G 
│ └─md0                  LVM2_member          50G 
│   ├─system-swap        swap                 10G [SWAP]
│   └─system-root        ext4                 40G /
├─sdb3                   linux_raid_member 881,5G 
│ └─md1                  LVM2_member       881,4G 
│   ├─lxc-hosting        ext3                450G 
│   ├─lxc-ns1-real                             1G 
│   │ ├─lxc-ns1          ext3                  1G 
│   │ └─lxc-ns1--snap6   ext3                  1G 
│   └─lxc-ns1--snap6-cow                       1G 
│     └─lxc-ns1--snap6   ext3                  1G 
└─sdb4                   ext4              931,5G 
sdc                      linux_raid_member 931,5G 
└─md2                    LVM2_member       931,4G 
  └─reserve-backups      ext4              931,4G /var/backups/mounted
sdd                      linux_raid_member 931,5G 
└─md2                    LVM2_member       931,4G 
  └─reserve-backups      ext4              931,4G /var/backups/mounted

```

Output of lsscsi:

```
[0:0:0:0]    disk    ATA      HGST HUS726020AL W517  /dev/sda 
[1:0:0:0]    disk    ATA      HGST HUS726020AL W517  /dev/sdb 
[2:0:0:0]    disk    ATA      WDC WD1002FBYS-0 0C06  /dev/sdc 
[3:0:0:0]    disk    ATA      WDC WD1002FBYS-0 0C06  /dev/sdd 

```

I have performed new test with pv (see [Load with pv < /dev/zero > /dev/md2](https://i.stack.imgur.com/FTfjs.png)). Now the situation is better. Operation on disks sdc and sdd still affects performance of sda, but load average keeps on 6-7 and the system is not overloaded. And I think now I understand what may be the reason of the issue. Since sda and sdb (as parts of RAID) used for webserver, they have constant stream of parallel requests. Hard disks have to constanly move their heads thereby reducing IO performance, but the system IO cache (I believe linux uses all free memory for this purpose) helps to minimize the number of disk accesses. When I begin to intensively use other disks, the cache becames filled with new data, that is why this increases the load on the main disks. If I'm right, increasing the amount of RAM will help. Now it's only 16 GB.