SSD TRIM on balance

SSD TRIM on balance

[Sergey E. Kolesnikov]

Greetings everyone.

I've recently rebalanced my btrfs (kernel 3.13) running on two SSD 
drives (raid-0) and a question arose.
Does btrfs perform TRIM on SSD balance? Fs is mounted with 'ssd' option, 
but not 'discard'.

It seems fstrim only trims free chunks in allocated space, so we 
possibly having a "trim leak" if no special treatment is taken on blocks 
reallocated during balance operation.

root@midnight:/mnt/tmp# fstrim -v /
/: 0 bytes were trimmed


root@midnight:/mnt/tmp# btrfs fi df /
Data, RAID0: total=90.00GiB, used=86.84GiB
System, RAID1: total=32.00MiB, used=12.00KiB
Metadata, RAID1: total=4.00GiB, used=2.56GiB


root@midnight:/mnt/tmp# btrfs fi show /
Label: ssdbtrfs  uuid: f61f749b-dce6-4e73-9d53-693e93ad61aa
     Total devices 2 FS bytes used 89.41GiB
     devid    1 size 111.31GiB used 49.03GiB path /dev/sda2
     devid    2 size 111.31GiB used 49.03GiB path /dev/sdb2

Btrfs v3.12


Best regards,
Sergey E. Kolesnikov

Re: SSD TRIM on balance

[Duncan]

Sergey E. Kolesnikov posted on Fri, 18 Apr 2014 21:07:00 +0400 as
excerpted:

> I've recently rebalanced my btrfs (kernel 3.13) running on two SSD
> drives (raid-0) and a question arose.
> Does btrfs perform TRIM on SSD balance? Fs is mounted with 'ssd' option,
> but not 'discard'.
> 
> It seems fstrim only trims free chunks in allocated space, so we
> possibly having a "trim leak" if no special treatment is taken on blocks
> reallocated during balance operation.

As a user and list-regular, I don't know the answer to your question as 
I've not seen it come up before and don't recall seeing it covered on the 
wiki or the like either, but you have a very good point... and question.

FWIW I'm on dual-physical-device-SSDs here too (tho on btrfs raid1 mainly 
because I like having a second copy of everything to restore from in case 
the one copy goes bad and fails checksum) and also don't use the discard 
option.  While I'm currently running nearly 50% overprovisioned and thus 
the FTL has plenty of space to do its thing regardless of what I do with 
trim on the actually partitioned space, you raise a very good question 
indeed, one I wish I had thought to ask myself, and now that you have, 
one I'd very much like an answer to as well.

=:^)

-- 
Duncan - List replies preferred.   No HTML msgs.
"Every nonfree program has a lord, a master --
and if you use the program, he is your master."  Richard Stallman