On 9/11/16, Duncan <1i5t5.dun...@cox.net> wrote: > Martin Steigerwald posted on Sun, 11 Sep 2016 17:32:44 +0200 as excerpted: >>> What is the smallest recommended fs size for btrfs? >>> Can we say size should be in multiples of 64MB? >> Do you want to know the smalled *recommended* or the smallest *possible* >> size?
In fact both. I'm reconsidering my options for /boot > * 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. (!!) > * There's also a system chunk to consider. This too is normally dup mode > on single device, raid1 on multi. While it shrinks to some extent with > size of filesystem, my 256 MiB /boot still has a 15.5 MiB system chunk, > doubled due to dup mode to 31 MiB. This is overhead you won't be able to > use for anything else. > > * There's also global reserve. This is a part of metadata (and thus > mixed-mode) that cannot be used under normal circumstances either. > However, unlike normal metadata, the accounting here is single -- it's X > space reserved no matter the replication type. On my 256 MiB /boot, it's > 4 MiB. > > So of my 256 MiB btrfs mixed-mode /boot, 31+4=35 MiB is overhead, leaving > 221 MiB for actual data and metadata. But due to dup mode that's halved, > to 110.5 MiB usable space. That's quite an info.. Thanks a lot.. Just to note again: Ordinary 127MB btrfs gives "Out of space" around 64MB payload. 128MB is usable to the end. I'm experimenting with an extracted (and customized) initrd on /boot. That is, /boot is a "minimal root on its own" which can switch to real root or do other things. Kernel and modules at /boot will not support any fs other than btrfs. (Or it may) It seems a minimally usable root around 10MB is possible. And it is free of udev and systemd.. -- To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
