Re: Small fs

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

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.