Slow disk write with mdadm

[GattoDispettoso]

Yes, i know mdadm is not recommended but it should work.

My HP N54l microserver has 8GB ram and 2x2TB WD red hdd.
Pxomox 3.1.21 Linux 2.6.32-26-pve
First installed on sda reserving a large partition for my data, then configured raid1 with mdadm.
All seems OK: server load,io wait, cpu usage.
VM's (all KVM with windows XP) show correct values but all are very slow.

finally i found that access to disk is tremendously slow when writing: often led lit fixed and very bad speed.

I think that this is not due to the 4K I/O size as i think the hd are aligned as required

Disk /dev/sda: 2000.4 GB, 2000398934016 bytes
255 heads, 63 sectors/track, 243201 cylinders, total 3907029168 sectors
Units = sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 4096 bytes
I/O size (minimum/optimal): 4096 bytes / 4096 bytes
Disk identifier: 0x000bad1d

Device Boot      Start         End      Blocks   Id  System
/dev/sda1   *      2048     1048575      523264   fd  Linux raid autodetect
/dev/sda2         1048576  1048575999   523763712   fd  Linux raid autodetect
/dev/sda3      1048576000  3907029167  1429226584   fd  Linux raid autodetect

i tried to run VM's from /dev/md2 (/dev/sda3 /dev/sdb3) instead of the standard partition with worse results.

Any suggestion?

Thank you
Fabrizio

 

[sdinet]

I don't know anyone who has good things to say about software-based RAID.

 

[matthew]

I am not sure about using it with proxmox but I have used software raid mdadm on Centos servers over years and it has always worked great. Have also used it on proxmox 3.x and it seemed fine. With ZFS in proxmox 4.x I guess it is no longer necessary.

 

[GattoDispettoso]

yesterday, after posting, i made a new test:
microserver with only 1 HD 2TB WD WD20EZRX
installed standard proxmox /version as above) from CD and upgraded
Restored a VM
result: very slow disk access not as with mdadm (of course) but still very slow .
So, this is not an mdadm ussue.