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OS: Ubuntu 14.04 LTS Software Raid10: 4x120Gb Intel 530 SSDs Server: HP ProLiant DL160 G6 FS: md0: ext4/ md1:swap

Test is made on fresh new install, before making any change to some package or reboot.

Writing speed decreased to the a normal SSD write speed (in my case).

Bellow I add the stats/commands I performed.

dd if=/dev/zero of=test bs=1048576 count=2048
2048+0 records in
2048+0 records out
2147483648 bytes (2.1 GB) copied, 1.79013 s, 1.2 GB/s

dd if=test of=/dev/null bs=1048576
2048+0 records in
2048+0 records out
2147483648 bytes (2.1 GB) copied, 0.376622 s, 5.7 GB/s

Everythig is looking good so far. Now I took out and put back in one of the disks:

mdadm --fail /dev/md0 /dev/sda1
mdadm --remove /dev/md0 /dev/sda1
mdadm --fail /dev/md1 /dev/sda2
mdadm --remove /dev/md1 /dev/sda2
mdadm --add /dev/md0 /dev/sda1
mdadm --add /dev/md1 /dev/sda2

After rebuilding the array:

dd if=/dev/zero of=test bs=1048576 count=2048
2048+0 records in
2048+0 records out
2147483648 bytes (2.1 GB) copied, 9.5621 s, 225 MB/s

dd if=test of=/dev/null bs=1048576
2048+0 records in
2048+0 records out
2147483648 bytes (2.1 GB) copied, 0.375301 s, 5.7 GB/s

My arrays look like this:

cat /proc/mdstat
Personalities : [linear] [multipath] [raid0] [raid1] [raid10] [raid6] [raid5] [raid4]
md0 : active raid10 sdc1[2] sdb1[1] sda1[4] sdd1[3]
195177472 blocks super 1.2 512K chunks 2 near-copies [4/4] [UUUU]

md1 : active raid10 sdd2[5] sda2[4] sdc2[2] sdb2[1]
39095296 blocks super 1.2 512K chunks 2 near-copies [4/4] [UUUU]

I don't know where to start now. Do I miss to set some flag or something?

1 Answer 1

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I'm not sure if all of this is correct, but:

  • mdadm syncs blocks when rebuilding an array, which should be identical to writing to the entire partition or disk of the newly replaced drive.
  • Using SSDs storage capacity beyond a certain percentage impacts their performance.

So because rebuilding your arrays caused writing at least one SSD beyond that percentage or until it's full may have caused this performance degradation.

Running a manual trim job on the devices may fix this, but could lead to data loss. Also copying zeros isn't a reliable method for measuring SSD performance.

You should have a look at how Btrfs or ZFS handle such cases. They should be more suited for setting up RAID sets with SSDs.

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