On 2016-05-15 08:12, Ferry Toth wrote:
Is there anything going on in this area?
We have btrfs in RAID10 using 4 HDD's for many years now with a rotating
scheme of snapshots for easy backup. <10% files (bytes) change between
oldest snapshot and the current state.
However, the filesystem seems to become very slow, probably due to the
RAID10 and the snapshots.
While it's not exactly what you're thinking of, have you tried running
BTRFS in raid1 mode on top of two DM/MD RAID0 volumes? This provides
the same degree of data integrity that BTRFS raid10 does, but gets
measurably better performance.
It would be fantastic if we could just add 4 SSD's to the pool and btrfs
would just magically prefer to put often accessed files there and move
older or less popular files to the HDD's.
In my simple mind this can not be done easily using bcache as that would
require completely rebuilding the file system on top of bcache (can not
just add a few SSD's to the pool), while implementing a cache inside btrfs
is probably a complex thing with lots of overhead.
You may want to look into dm-cache, as that doesn't require reformatting
the source device. It doesn't quite get the same performance as bcache,
but for me at least, the lower performance is a reasonable trade-off for
being able to easily convert a device to use it, and being able to
easily convert away from it if need be.
Simply telling the allocator to prefer new files to go to the ssd and
move away unpopular stuff to hdd during balance should do the trick, or am
I wrong?
In theory this would work as a first implementation, but it would need
to have automatic data migration as an option to be considered
practical, and that's not as easy to do correctly.
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