Very bad write speed on mdadm RAID1 with SSDs

DerFlo129 asked:

I have two Samsung 860 QVO in RAID1 with mdadm. The write speed of a single disc is around 450MB/s. I reach a maximum of 70MB/s on the RAID 1. The reading speed is normal.

xfs is used as the file system.

What is the problem here?


Personalities : [linear] [multipath] [raid0] [raid1] [raid6] [raid5] [raid4] [raid10]
md0 : active raid1 sdb[0] sdc[1]
      976630464 blocks super 1.2 [2/2] [UU]
      bitmap: 3/8 pages [12KB], 65536KB chunk

unused devices: <none>

dd if=/dev/zero of=/vm-storage/testfile bs=1G count=1 oflag=dsync

1073741824 bytes (1,1 GB, 1,0 GiB) copied, 16,6422 s, 64,5 MB/s

My answer:

Your writes are slow because you used oflag=dsync. As the man page states:

       dsync  use synchronized I/O for data

This means that the system waits after every write until the storage has confirmed it is written. Generally you wouldn’t use synchronized I/O unless your application desperately needed to ensure that a single write was actually committed to the media before continuing.

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