On Wed, Feb 15, 2012 at 10:42:43AM -0500, Calvin Walton wrote:

On Sun, 2012-02-12 at 16:14 -0800, Marc MERLIN wrote:

quoted

Considering that I have a fairly new crucial 256GB SDD, I'm going to assume
that this bit applies to me:
"On the other side, TRIM is usually overrated. Drive itself should keep good
performance even without TRIM, either by using internal garbage collecting
process or by some area reserved for optimal writes handling."

So it sounds like I should just not give the "ssd" mount option to btrfs,
and not worry about TRIM. 

The 'ssd' option on btrfs is actually completely unrelated to trim
support. Instead, it changes how blocks are allocated on the device,
taking advantage of the the improved random read/write speed. The 'ssd'
option should be autodetected on most SSDs, but I don't know if this is
handled correctly when you're using dm-crypt. (Btrfs prints a message at
mount time when it autodetects this.) It shouldn't hurt to leave it.

 
Yes, I found out more after I got my laptop back up (I had limited search
while I was rebuilding it). Thanks for clearing up my improper guess at the
time :)

The good news is that ssd mode is autodetected through dmcrypt:
[   23.130486] device label btrfs_pool1 devid 1 transid 732 /dev/mapper/cryptroot
[   23.130854] btrfs: disk space caching is enabled
[   23.175547] Btrfs detected SSD devices, enabling SSD mode

Discard is handled with a separate mount option on btrfs (called
'discard'), and is disabled by default even if you have the 'ssd' option
enabled, because of the negative performance impact it has had on some
SSDs.

That's what I read up later. It's a bit counter intuitive after all the work
what went into TRIM to then figure out that actually there are more reasons
not to bother with it then to do :)
On the plus side, it means SSDs are getting better and don't need special
code that makes data recovery harder should you ever need it.

I tried updating the wiki pages, because:
https://btrfs.wiki.kernel.org/articles/f/a/q/FAQ_1fe9.html
says nothing about
- trim/discard
- dmcrypt

while
https://btrfs.wiki.kernel.org/articles/g/o/t/Gotchas.html
still states 'btrfs volumes on top of dm-crypt block devices (and possibly
LVM) require write-caching to be turned off on the underlying HDD. Failing
to do so, in the event of a power failure, may result in corruption not yet
handled by btrfs code. (2.6.33) '

I'm happy to fix both pages, but the login link of course doesn't work and
I'm not sure where the canonical copy to edit actually is or if I can get
access.

That said if someone else can fix it too, that's great :)

Thanks,
Marc
-- 
"A mouse is a device used to point at the xterm you want to type in" - A.S.R.
Microsoft is to operating systems ....
                                      .... what McDonalds is to gourmet cooking
Home page: http://marc.merlins.org/