Re: BTRFS, SSD and single metadata

[Austin S Hemmelgarn <ahferroin7@gmail.com>]

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On 2014-06-16 03:54, Swâmi Petaramesh wrote:
> Hi,
> 
> I created a BTRFS filesytem over LVM over LUKS encryption on an SSD [yes, I 
> know...], and I noticed that the FS got created with metadata in "DUP" mode, 
> contrary to what "man mkfs.btrfs" says for SSDs -> it would be supposed to be 
> "SINGLE"...
> 
> Well I don't know if my system didn't identify the SSD because of the LVM+LUKS 
> stack (however it mounts well by itself with the "ssd" flag and accepts the 
> "discard" option [yes, I know...]), or if the manpage is obsolete or if this 
> feature just doesn't work...?
> 
> The SSD being a Micron RealSSD C400
> 
> For both SSD preservation and data integrity, would it be advisable to change 
> metadata to "SINGLE" using a rebalance, or if I'd better just leave things the 
> way they are...?
> 
> TIA for any insight.
> 
What mkfs.btrfs looks at is
/sys/block/<whatever-device>/queue/rotational, if that is 1 it knows
that the device isn't a SSD.  I believe that LVM passes through whatever
the next lower layer's value is, but dmcrypt (and by extension LUKS)
always force it to a 1 (possibly to prevent programs from using
heuristics for enabling discard)


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