I picked up a SSD (30GB OCZ Vertex if you're curious) and am planning to set up a RAID1 with it and a SATA disk with the SATA disk in write-mostly mode along with write-behind enabled. So I went to test it to make sure I had all the details correct on aligning the partition to a 128kB offset and the correct mdadm commands to create the array in write-mostly and write-behind, but I found that I couldn't get writes to "write-behind". Essentially all writes would complete at the speed of the rotating disk. Reads however worked great - they all appeared to be coming from the SSD. I created the array with the following command: mdadm --create /dev/md0 --raid-devices=2 --level=1 \ --bitmap=internal --write-behind /dev/sdb1 --write-mostly /dev/sda1 /dev/sda is the SATA disk, /dev/sdb is the SSD. So then I created an ext3 filesystem on the array (mkfs -t ext3 -E stride=32 /dev/md0), mounted it and proceeded to run some quick dd tests to verify that things were working as expected. I simply wrote a 1kB file to disk like this: dd if=/dev/zero of=/mnt/disk/tmpfile bs=1k count=1 conv=fdatasync I tried various file sizes, and even tried reconfiguring the bitmap with a write-behind setting of 10000. I confirmed speeds by failing and removing either the SSD or SATA disk - with both disks active, writes were completing at the speed of the SATA disk, which was 2-5 times slower depending on the size of the file. My only guess is that by specifying fdatasync on the dd command, that forces the data to be written out to all members of the array. Not quite intuitive if that is by design - because otherwise, when are you able to take advantage of the write-behind feature. The system I was testing on was running off a Fedora 10 live CD, so it's not the most recent software, but it's not that old, either: (kernel 2.6.27.5-117.fc10, mdadm 2.6.7.1). Can anyone confirm that write-behind is working for them, and a quick test to show that it's working? -Dave -- To unsubscribe from this list: send the line "unsubscribe linux-raid" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
