On Sun, 16 Dec 2012, cwillu [off-list ref] wrote:

3.2 is massively old in btrfs terms, with lots of fun little stability
and performance bugs.

quoted

Here is the /proc/mounts entry which shows that ssd and discard options
are enabled.

/dev/disk/by-uuid/7939c405-c656-4e85-a6a0-29f17be09585 / btrfs
rw,seclabel,nodev,noatime,ssd,discard,space_cache 0 0

Don't use discard; it's a non-queuing command, which means your
performance will suck unless your device is really terrible at
garbage collection (in which case, it's just the lesser of two evils).

Thanks for the advice.  On one of my systems a reinstall of the linux-
image-3.6-trunk-amd64 package went from almost 4 minutes to only 29 seconds 
when I removed the discard option.  When I then booted the 3.6.9 kernel in 
question the package install time dropped to 27 seconds (of which more than 
half was CPU time).  I only did one test run of each, so 27s vs 29s could be 
within the range of random variation but 4 minutes vs <30s is an obvious and 
massive improvement.

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