On 2016-09-11 15:51, Martin Steigerwald wrote:
Am Sonntag, 11. September 2016, 19:46:32 CEST schrieb Hugo Mills:
On Sun, Sep 11, 2016 at 09:13:28PM +0200, Martin Steigerwald wrote:
Am Sonntag, 11. September 2016, 16:44:23 CEST schrieb Duncan:
* Metadata, and thus mixed-bg, defaults to DUP mode on a single-device
filesystem (except on ssd where I actually still use it myself, and
recommend it except for ssds that do firmware dedupe). In mixed-mode
this means two copies of data as well, which halves the usable space.
IOW, when using mixed-mode, which is recommended under a gig, and dup
replication which is then the single-device default, effective usable
space is **HALVED**, so 256 MiB btrfs size becomes 128 MiB usable. (!!)
I don´t get this part. That is just *metadata* being duplicated, not the
actual *data* inside the files. Or am I missing something here?
In mixed mode, there's no distinction: Data and metadata both use
the same chunks. If those chunks are DUP, then both data and metadata
are duplicated, and you get half the space available.
In german I´d say "autsch", in english according to pda.leo.org "ouch", to
this.
Okay, I just erased using mixed mode as an idea from my mind altogether :).
Until recently (when DUP mode for data on a single device got added), it
did have advantages. If you're running on a single disk and need
replication, it's a lot more efficient to just run a single partition in
DUP mode than two in RAID1 mode. It also makes it a bit less likely to
hit ENOSPC issues on small filesystems.
Just like I think I will never use a BTRFS below 5 GiB. Well, with one
exception, maybe on the eMMC flash of the new Omnia Turris router that I hope
will arrive soon at my place.
FWIW, I don't use anything smaller than 8G unless I have to (with the
notable exception of /boot), and when I do need to, I usually just use
mixed mode because once you get that small, you need all the help you
can get to keep from the whole FS being allocated to chunks.
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