Hello everyone,

A quick update on the btrfs SSD modes. The pull request I just to Linus includes autodetection of ssd devices based on the queue rotating flag. You can see this for your devices in /sys/block/xxx/queue/rotating

If all the devices in your FS have a 0 in the rotating flag, btrfs automatically enables ssd mode. You can turn this off with mount -o nossd.

The default ssd flag tries to find rough groupings of blocks to allocate on, and will try to pack blocks into the free space available. So, if you have something like this (pretending a bitmap of free blocks)

free | free | used | free | used | free | free | used

Btrfs SSD mode will collect a large region of mostly free blocks and allocate from that. This works well on newer and high end ssds that prefer us to reuse blocks instead of spreading IO across the hole device.

But, low end devices may have to do a read/modify/write cycle when we actually do IO in this case. I've added a new mount option for those devices:

mount -o ssd_spread. This is not autodetected, you still need to pass it on the mount command line or in /etc/fstab. In ssd_spread mode, btrfs will try much harder to find a contiguous chunk of free blocks and hand those out.

-chris