On Thu, Mar 26, 2009 at 9:29 PM, Neil Brown <neilb@xxxxxxx> wrote: > On Wednesday March 25, drees76@xxxxxxxxx wrote: >> 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 > > Hmmm... try with an external bitmap stored elsewhere on the SSD. > When you have bitmap=internal, bitmap updates are synchronous to all > devices. Maybe that is causing the loss of speed that you see. Darn, I gave that a brief thought when I was setting it up but later forgot about it. I'll have to give it another test. If so, we should probably include some docs alongside the write-behind information noting that you need to put the bitmap externally to see any benefit. Hopefully I'll have time to get this set up and tested over the next couple days. -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
